I thought I asked this already but I have the latest and greatest sonar.  Two different hardcore windows rigs, one windows 7 one windows 10.  The windows 10 rig has 32 GB ram , the latest 6 core processor and solid state drives.  
 
On projects with more than two or three software instruments ,  some of the piano will simply not play or some drum hits ill simply not play.  if you rewind and play back,  different notes will not play.
 
It seems to be related to ..processing somehow, I have tried different buffer sizes and it does not seem to matter which audio hardware I use, RME or FOCUSRITE.   
 
Its like instead of an audio dropout, it is dropping out part of the audio to keep from fully dropping out. 
 
 
I would kind of prefer that it just dropped out.  Is this supposed to happen? is it how sonar is handling dropouts now?
 
I should also add that ,  I would not expect ANY dropouts with this cutting edge of a machine? at these settings

try increasing the "Prepare using nnn millisecond buffers" from 250 to 500 or 750 in Preferences > MIDI > Playback and Recording

Yes that was it I believe. First test played everything , I will try on the other rig.  I was not familiar with that setting. But I also had Dejavu, as I wondered if I had asked this already but could not find my post. Anyway thanks sir!

For reasons I never understood, with either X1 or X2, they changed the default from 500 down to 250 for the Prepare Using buffers, and some folks began having exactly your issue, so for folks getting bit by that, increasing it back to either 500 or even up to 750 can help eliminate this.
 
Bob Bone