Disconnect the YC from anything MIDI or USB, completely eliminate a situation where it is caused by something external to the YC.
If it never happens when you disconnect, then you can focus your attention on your external connection via MIDI.
If it happens again, then start documenting (exactly) which Keys, which programs, details.

(Details like “...it happens on many keys” is not really specific enough to make a guess
When connected to something external (we assume the App is hosted by some kind of hardware device) does the device “go to sleep”? And the first note-on wakes it up? You give us no details on the App besides it’s name... I’m not familiar with it or even what it runs on. But before we look there... try to reproduce the issue on the YC standalone.

Let us know

Ok, removing the iPad with camera USB connection makes no difference.
It seems to be only possible with pianos. If I engage the organ it always triggers a sound (kind of ruling out a mechanical issue)
And I can now reproduce it on every key.

It looks like sometimes when you press the key slightly and then press harder there is no sound.
A CFX tends to react differently as for instance the S700. The CFX makes it easier to reproduce the sound.

Is this something with the velocity recognition.
If I press half way and press fast and hard there is no sound. An organ produces a sound halfway already.
If I select "touch" to hard it always gives sound but the expression is gone. Always loud.

Does this mean i do something wrong when practicing?
Thanks

Does this occur while you are *playing* the piano or are you trying to send a velocity of 1
On an acoustic piano - if you press a note as softly as you can, it will not sound.
If you play normally does it play normally or are you trying to “fool the system” by trying to hit is as softly as possible?

Some programmers love to make velocity range on acoustic piano go from 2-127, so that those trying to press notes as lightly as possible will discover that, yes, you can engage a silent note. I don’t try to fool the system - and I never see any reason for hitting a key slightly like that.

I don’t know exactly what this means: “If I press half way and press fast and hard there is no sound.”

If you are looking for triple sensors, you needed to get the YC88 — it has three sensors on the individual key travel, so you can press a key key halfway or even 2/3 of the way down, the press hard to trigger a note... because there are more sensors on the way down.

Did you get the wrong model? There are three YC models, the YC61 is for the organ purist (waterfall keys are essential to truly speak the organ vocabulary). The YC73 is in between acoustic piano and organ, while the YC88 is for the die-hard pianist — it naturally has the most acoustic piano action!

YC88 NATURAL WOOD WITH TRIPLE SENSOR ACTION
The natural wood keys and graded action of the YC88 reproduce the weight and response of a grand piano right down to the feel of the heavy or light hammers in the low and high ends. YC88 also features a triple sensor action for pianists accustomed to the response and speed of grand pianos. You get the touch of a Yamaha acoustic piano with the unmatched versatility of the YC.

I was wondering what the 3x sensor meant but honestly for 2K (YC61) and 2.5K (YC73) shouldn't the keys make a sound without needing to pay the premium price for an additional number of sensors?

Shouldn't a "stage" classification automatically mean you get more sensors? You get less or no synth so why not more sensors?

I know. Other manufacturers don't offer this either. I'll have to check mine out to see if it doesn't make any sound when starting from a partially pressed position.

I know my YC73 felt much better, not sure if it behaves better than the 61 but I did own both.

I think it starts to make sense, but my playing skills (or lack of it) trigger this behaviour.

@Bad Mister:
So to answer some questions:
- yes it happens when playing, and I can reproduce it when pressing the key with very low velocity.
- No I didn't get the wrong model, I went for the organ " feel" of the YC61. When a week after my purchase of the YC61 the YC73 and 88 were introduced I could return the 61 and get a 73 but I didn't want to lose the waterfall keybed for the ease of sliding (organ centric) the keybed.
- Indeed if you take an acoustic piano you need some velocity to ge a tone, and I think that is what is emulated here and what my playing triggers.
- pressing half way means probably that I am past the sensor, and any speed I add then isn't "seen" by the YC61.
- According to Yamaha: The authentic Waterfall keyboard action of the YC61 was created by painstaking research into traditional organ keyboards and constant feedback from top keyboard artists. I trust they did their homework but have no knowledge to confirm that.
- So it never happens in organ mode, and never in "hard" mode of the touch setting.
- The YC73 and YC88 have the keybeds of the CP73 and CP88 respectively only the YC61 has this waterfall organ type keybed.
- I think the YC61 only reacts to velocity and not pressure
- As you and Yamaha states YC61 for organ centric, YC73 for electric piano centric and YC88 for acoustic piano centric players but the share the same engine.
- I played a Yamaha Clavinova today and felt the massive difference of full weighted piano keys, and a different behaviour all together.

@David
You owned both the YC61 and 73? (or was it the CP73?) can you tell more?
I have ordered a CP73 as second manual to put under the YC61 (yes overkill, but luckily I do not have to live from making musing as I would be a poor bastard :p )
So I will connect the CP73 via MIDI to the YC61 for a B3 emulation, analog cascade them so I can also play the CP73 when practicing with piano sounds and I guess that keybed is better suited for that.
The CP88 is to big for me and I am not keen on a "real" piano like feel as I never played one.

Thanks for your replies.