In short:
 
I have a separate instance of Addictive drums open in X3. I usually record via my DTX900. I open the staff view for this particular track. When dragging a note up and down the note grid you hear the drums as usual. When I open the GUI for AD and hit the various surfaces everythings sounds. But when I in staff view write down a simple count-in for the drums I'm about to lay down, they do not sound during playback. First time this has happened, and I CANNOT figure out what is wrong. Again - I put a simple one-bar count-in with the Cowbell. When I place it on the grid everything sounds. When rewinding a bar and playback - complete silence. Input?

Increase your MIDi Prepare Using buffer to 500ms under Preferences > MIDI > Playback and Recording. The new lower default of 250 is too low for many systems.
 
 

brundlefly
Increase your MIDi Prepare Using buffer to 500ms under Preferences > MIDI > Playback and Recording. The new lower default of 250 is too low for many systems.
 
 

Thanks - but that is one of the first things I try should these things occur. Without success obviously.
Any other suggestion? 
 
My system should be fairly capable I believe, even if it's a few years old by now, with i7 2.93 GHz, 16 GB RAM, 4 separate WDBlack 7200 drives and a little RME UCX interface to bond with. 

So notes recorded from your controller play back okay, and notes are heard as you enter them in the Staff View but do not play back properly? What about entering notes in the PRV? Do those play back? If so, check the Event List view for notes entered via the staff view and the PRV, and see if there is a difference.
 
 

It is sometimes hard to tell that you've accidentally muted a clip. This seems to suit your symptoms.

worstcaseontario
It is sometimes hard to tell that you've accidentally muted a clip. This seems to suit your symptoms.

Thanks - but not that either. To recap:
 
I have a plethora of tracks in the project (backing tracks for live performance). Even if they're all kontakt, the playback is stellar, but for this case I archive everything just to zone in on why Addictive Drums won't play.
 
- I can hear the sound in real-time just fine when I play it on my DTX900.
- When I open AD GUI and hit the drums, they sound excellent, just as they should.
- When I open staff view - AND piano roll view - and drag up and down I also get the full spectrum of sounds in realtime.
 
But when entering in notes in any of them and playback - no sound. Every other track in the project sounds fine though. And the clip is NOT muted.
It's as if has decided to not read that particular track only. 
 

Sounds like your MIDI track might not be pointing to AD?  I know this sounds simple but the MIDI track output needs to be set to AD... the midi channel needs to be set as well?
 
Jim
 

Also check to see which Midi channel AD is "listening" to, and which channel the notes are being entered/recorded into

It's an instrument track. The synth (AD) and the midi track are joined.

Lane muted and hidden? Real-time input will still be echoed, but recorded/step-entered MIDI will not play back, and there will be no indication anywhere that the lane is muted.