I'm guessing it is a deficiency in adequate polyphony because some of the parts are FM-X?

Not really. Although FM-X has less total "notes" of polyphony - FM-X is more efficient overall in using its resources. Parts 2-6 use 5 units of polyphony for every key pressed primarily in the "left hand" (below F#3 due to the split). If you play 5 notes in the left all below F#3 then at most this is going to consume 25 "notes" of polyphony with no overlap. Overlap can occur if you hold the sustain pedal down and the notes do not decay. Here you could play 2 sustained 5-note chords before the 3rd runs out.

The AWM2 part has the possibility (although probably not in your Performance) of having a single Part consume more polyphony per key pressed than FM-X can. While FM-X does consume 5 units of polyphony under F#3 - the 1st Part could, hypothetically, all by itself consume 8 units of polyphony (under G#5 only).

So yes, there's areas of the keyboard where you could play and FM-X could reach its polyphony count faster than AWM2. I don't know how many elements the AWM2 Part can use at once - but given it's labeled as a piano - it probably only uses one per piano key pressed.

Generically, what I am letting you know is that just because FM-X makes available 1/2 the polyphony maximum than AWM2 - the fact that you mix AWM2 and FM-X does not necessarily mean FM-X will run out first. Even if you had all of your Parts set to full keyboard limits (C-2 to G8). But it really depends on the element utilization of the AWM2 part. FM-X doesn't require any further information to determine how it operates per in-range keyboard key pressed.

Note that when you run into polyphony or note (not polyphony related) limits - you'll generally see the meters on the [PERFORMANCE] (HOME) page drop down to zero for that Part that has had its notes "stolen". If FM-X is dropping off, you'll probably see this in Part 6 first because Part 6 has the lowest priority.

Since AWM2 and FM-X are completely independent when it comes to polyphony - you can turn off keyboard control for Part 1 (then press [PERFORMANCE] (HOME) so Part 1 is no longer selected) and play without AWM2 so you can focus on what's happening with FM-X. You'll know it's FM-X dropping by listening and would not need to look at the meters. Likewise, you could turn on Part 1's keyboard control and turn off Parts' 2-6 keyboard control and again press [PERFORMANCE] (HOME) to make sure no Part is selected. Then play again and if you hear dropped notes you know it is AWM2 (or also AWM2 depending on if FM-X dropped too).

If you play two hands above F2 - then it isn't FM-X dropping because FM-X isn't triggered above F2. Pay attention to the range of your notes and how that relates to your splits. Just so you know how that relates to whats sounding.

Sustain and release are similar. So the sustain pedal (leaving it down) can eat up polyphony as well as a long release time on notes. Take a look at your AEGs and see if release (notes continued after letting go of the piano keys) could be part of the consumption here.

Thank you for your detailed reply, Jason! Very kind of you. 1. So how do I avoid dropping notes due to a polyphony issue? 2. Could note-dropping be related to any other issue that I should address?

If you hold the sustain pedal down (or have long releases on either engine - FM-X or AWM2) then press the same note more than 4 times then you'll have dropped notes too.

I would figure out if it's Part 6 dropping notes primarily and then decide if Part 6 is really the least important or not. If there's another Part that isn't as critical then maybe swap places with Part 6 and the other (from 2-5). If this is a polyphony issue (which it isn't exactly established yet - I'm just assuming for now it is) then you may want a different Part to have notes "stolen" instead.

If this is because of sustain (use of the pedal) - then maybe you can turn sustain receive off from one or more of the Part(s) 2-6. I imagine the piano (Part 1) you'd want to leave it on.

Also take a look at your release levels and times. If those are set too large they can be problematic.

@Jason

Does fudging longer releases by using Delays and Reverbs save Polyphony count by permitting shorter Releases with a similarly longer sound?

To me, it feels like the Reverbs are cumulative, and a buffer of their own, but I haven't tested to see if this is the case.

This may or may not apply, but I mention it as a "be aware" scenario.

This falls under "Good House-Keeping".

Very often I will "borrow" Parts from Factory Performances when creating my own.

All those Factory Performance/Parts were created by different Human's who employed different methods to achieve the same end results (testament to the power of the MODX OS).

When you put all those different parts together, wierd things can happen. For example, no sound from a key press, where Keyboard Range indicates there should be sound.

The Keyboard Range for an AWM2 Part (e.g. C5 to D6) can be set per Part, AND per Element (Up to 8 Elements). Elements also have there own Velocity Ranges.

TIP#1
Make Sure the Ranges of all Elements sit within the "Master" Ranges set at the Part Level. The Part Ranges will "Mask Out" any Elements not set within that same Range.

Example: You find a Preset Part you would like to use... occasionally (e.g. xylophone). You decide to move (KB Range) the Part to the High End of the Keyboard to "keep it out of the way". But, you also decide, it is too high Pitch there so you add a "-24" Note Shift (Edit-Part Settings-Pitch-Note Shift).

Almost immediately you may experience what seems like No Sound or Polyphony Loss. Without jumping into details you will need adjust the Part KB Range to MATCH the Note Shift. You will also need to adjust all the Element KB Ranges to sit within the Part KB Range. I prefer to make the KB Range Limits match exactly, although this requires a little more effort.
While you are there check you don't have any Velocity mismatches either.

TIP#2
Check there is NO Keyboard Scaling (Amplitude) set for each Element. I notice some of the Preset Designers use this in Preference to setting KB Ranges. In effect they Set the KB Range with Volume = 127 in the desired KB range, and the Volume=0 for the rest of the KB. The Graph looks like a Square Wave. As far as I am aware this leads to very poor Note Polyphony (Volume = 0 is still using Polyphony) (Example of this on Preset "GOA PSYCHE").

Needless to say, the Element Amplitude Scaling will be enforced regardless of your set KB Ranges/Splits etc... so watch out for it.

TIP#3
Mod/Controls. Sometimes the imported PERFORMANCE or PART may still be governed by whatever MOD/CONTROL functions it was originally part of.

This means, sometimes, the PART/ELEMENTS do not sound or sound completely different than you expected.

The best I can offer here is, take the time to learn and understand the MODX MOD/CONTROL System, otherwise this post will become Bible sized.

Watch out for, in Particular, Element XA Control and Element Groups, also AF1&2 settings.

Mods & Control Assigns can be found in
Part Edit - Part - Common - Part Settings - Mod/Control... then any of Part LFO, Control Assign, Receive SW.

There are Radio Buttons you can press when doing Category Search for Part Import that allows you to import "without the strings attached". In writing this I forget the details but be aware.

Best rule of thumb I usually employ once I am happy I have the right sounds in my Performance is DELETE ALL Mod Controls and Assigns and rebuild them to my own needs and sense of logic. Again this requires deep knowledge/understanding of the Mod/Control system.

You will often find in many Presets, that most of the Assign Controls are just debris/junk "relics" that have been deactivated. These are "leftovers" from when the designer borrowed the Part from another performance and repurposed it for his use.

It would have been preferable for them to delete Assigns that weren't being used, otherwise they just serve to confuse the end user. But... time is money and all that. So, you have to do your own "Good House Keeping".

Effects (can) go on well after the elements have turned off and no longer take.

Take something like "Vintage Arp LFO". Turn off the arpeggiator so that isn't running. Edit Part 1's InsA. Change the feedback to +63.

You'll hear the effects just keep stacking and stacking since this setting will have the effects "take over".

If you take "All 9 Bars!" and place Part 1 in Parts 2-7 slots then this is a polyphony killer. 8x7 elements running at once. Place "Vintage Arp LFO" as Part 8 and adjust the InsA feedback. Then create the effect-generated loop by stacking up sounds. In this configuration Part 8 is the least priority polyphony Part. Now cram your arm down on the keys. You'll see most all of the Parts are not outputting because they have been polyphony cancelled. However, Part 8 is still "running" its effects loop. That shows it's no part of the polyphony count (effects).