last night I experienced a very strange issue. I use BFD2 for the drums. I was mixing and everything was working fine. Then I noticed one particular drum stopped working. All of the other drums were fine. I reloaded the drum sample, still didn't work. closed and reopened the project still didn't work. rebooted still didn't work. Opened a previous version of the same project and it worked just fine.  Went back to the project with the issue and deleted the midi notes and "re-drew" them back in, still didn't work. added a drum to BFD2 and moved the midi notes to the new drum(same sample) and it worked again. I looked at the event list and didn't see anything abnormal.  So it seems this is not a BFD2 issue. Almost like there is some corruption just on that one drum's midi notes. BTW I looked at all of the mix settings for that drum in BFD2 and it should be working. spent a lot of time trying to troubleshoot, way to much to list.
 
anyone have any ideas about this?

A few possibilities spring to mind:

• Were the offending kit piece's velocities set to 1, making it inaudible?
• Was the Midi channel changed from  the default?
• Was the track muted? (it seems like you checked this but it's worth asking)
• Was there any volume automation at either clip/track level?
• Was all your routing correct from track > bus > main outs
• Were you using a drum map? If so check the kit piece wasn't muted there (separate mute to track mute)
• In BFD, was the trim set correctly? Mixer fader set correctly? Output routed properly? 

I'm sure there are many more things that could cause this but these are the first things I'd check

when I playback the project, the drums (midi) plays normally as it should with the exception of just the one sample, up until then everything played back just fine with no issues.  The velocity is fine, none of the other items you mentioned are an issue.

imagine you programmed a drumbeat. You hear the hihat, kick, cymbals but you can't hear the snare.  you see there is a triggers for the snare, everything you see indicates you should be hearing a snare, but you don't. so you insert another drum into BFD2 and you assign that new drum sound to be a snare and move the midi notes to the new line in piano roll view and now you hear the snare. 

What happens if you load your original snare into a different slot in BFD?

You'll have to set up an additional track in Sonar to get it mapped properly, but we need to determine if it's Sonar or BFD

that is exactly what I did as mentioned above. It is definatly a sonar issue, i suspect somehow there is a "curruption" (lack of a better term) in the midi data storage in the project.  I have been recording several songs and for consitency I use the same "drumset" on all recordings. So all projects are using the same samples and settings. Everything is pretty much identical. No other project has any issues, just the one project. So I am pretty sure its not BFD. and since I inserted the same sample into another slot and it works now, makes me think there is an issue with Sonar.
 
So for now, I am pretty much done with the song/project and the work around is fine. But I am mostly curious and annoyed with this issue. it did side track me for an hour. I mostly worry it could happen again and be much worse of an issue without a work around in the future.  If you have BFD2, I could upload the project file without any audio tracks (just the midi drums) and you could see for yourself the behavior. Maybe I am missing something you might see right away.  It has dumbfounded me!  :)

Just a thought - in preferences - MIDI - Playback and recording - what is "prepare using ??? milliseconds buffers" set to?
 
Experiment with increasing it t see if problem goes away.
 
Mine is set to 500.

Did you try this?

i beleive it is set to 500. I did that a while ago.  I have also made some other changes, quite a while ago (a year maybe) to optimize it for my use. Have had very few issues since. I will double check tonight.

Perhaps try even higher, just to see if it sorts the problem for this one project.