With FM there are ways this would happen, but I don't know anything about your FM-X Bass part.

Which algorithm? Which operator number are you changing the envelope for? What's the raw level of the operator you're working with? If the level is 0 then the envelope is not going to make a difference. If you're changing the amplitude envelope for a modulator then this isn't going to change the output level either since a modulator changes timbre. The carrier (on the bottom row) could be used to affect the output level - and only those with a non-zero level.

There are other possibilities.

If there's a way to use Montage connect to save an X7B file, then zip and attach here - we can more quickly take a look. Or if you can upload the Performance to Soundmondo this would work too. Only the bass part needs to be included.

Or you can take pictures of the FM-X screens that show algorithm # and the various operator screens.

Also, there's a chance your times are too long compared to patience. Say 80 and above for time - you may not hear anything changing. At least not unless you wait for a while.

Use [CATEGORY SEARCH] to pull up the "Init Normal (FM-X)" Performance.

Now edit OP1 (operator 1) level where the AEG (amplitude envelope generator) is. Change Decay 1's level from 99 to 0 and Decay 1's time from 0 to 45. 45 is a reasonably fast - but not too fast - value for hearing the envelope. Play a note and hold it. You should be able to hear decay 1.

Instead of snapping back up to 99 instantly we can change decay 2 so it more gradually rises. Just change Decay 2's time to 45 (from 0). Now play the same note. You'll hear the fall and rise of the level better.

The point of doing this is just to establish that the parameters work and give some insight as to why your earlier experiments may not have panned (leveled) out.

@ivo...

#1 - Request you provide more info regards your problem.

#2 - Until you do #1, the below is an educated guess.

What I think you may be doing is....

a) Changing the Amp Envelope on a Modulator, that is not acting on the same Carrier you are listening too.

I suggest you start by looking at your Algorithm (and confirm the Algorithm Number here).

In my opinion, the best Algorithm for learning and understanding FM is Algorithm #67.

It looks like this...

1 3 5 7
2 4 6 8

This is 4x 2-Operator "Stacks".

2, 4, 6 and 8 are Carriers (Bottom Row)

1, 3, 5 and 7 are Modulators (Top Row)

1 is Modulating 2, 3 is Modulating 4 and so on.

1&2 are a single 2-Op Stack, 3&4 are a different Stack etc.

You can only hear a Stack if it's Carrier Level is turned up high... let's say = 99.

The Carrier is what you hear. Any changes you make to the Modulator, will only change the Timbre/Tone of the Carrier, not its Level (volume).

In Algo#67, Iets say Carrier 2 Level = 99. Carrier 4 = 0, Carrier 6 = 0 and Carrier 8 = 0.

So you can only hear Carrier 2 (it's volume is high).

If you made any changes to Modulator 3... you would not be able to detect those changes, because it's associated Carrier 4 is "off" (Level = 0, i.e. Volume = 0).

By comparison, if you changed the Level of Modulator 1, you would hear (detect) those changes in Carrier 2 because it's Level = 99 (Volume Up) . You would hear Carrier 2 change in Timbre.

Another Example... Algorithm #37

Looks like this

3
4
5
6
7
8 1 2

Ignore 1 and 2.

3, 4, 5, 6, 7 & 8 form a 6 Operator Stack, where Op8 is the Carrier, and all you can hear (Op8 Level = 99)

If Operator 7 (Modulator)
was set to Level = 0, then any changes you made to 3, 4, 5 or 6 would not be detected in Carrier 8. This is because Op7 Level = 0 is effectively "blocking" everything above it in the Stack.

b) The other reason you may not be detecting changes (Decay1, Decay2 etc) is that the changes are not "severe" enough to be detected.

This is hard to explain, so it's best to just experiment.

In Algo 67 --> Set Op2 (Carrier) Level = 99, and Set Op4, Op6 & Op8 Level = 0.

Now change Op1 (Modulating Op2) Level using the first Slider on the Top Panel. While playing a Note, If you move Slider 1 all the way from Bottom to Top and back again... you will hear the changes in Timbre. This is a "Severe" movement.

By comparison, if you move the Slider to around 75%, then move it back and forth between 70% 75%... you may not hear (detect) any change in Timbre. The Timbre IS changing, it's just so small you can't detect it.

This 70%-75% "small change" may be what is happening with your Decay 1/Decay 2 changes. Although the Decay 1/Decay 2 values may look large, they may only be causing small changes. This is because ADSR values are not Linear... which is hard to explain without mathematics.
.
Rule of thumb... change ADSR values while playing a Note so you can hear when they start to make changes.