If I a start a new project and insert VST and record midi its fine, just like last 15 years. 
 
But this existing almost completed project today just started not playing the Addictive Piano so I looked into it tried everything rebooted etc. Opened a version from a few days ago without closing sonar it works fine.  Something is up with this project? Switched the midi stuff back and forth even copied and recorded new midi and sent to new instance of synths, (any synth) the midi track registers on the meters, nothing on the synth output tracks and no synth output??
 
If I play live it sounds fine ( I can record live as well on the VST)  will not play the darn midi??
 
Anyone seen this? 
 

So when I export audio (render)  all the midi instatiated synth parts are there? so they are just not ...playing or previewing inside sonar at runtime?   Does Sonar now just ...not play stuff when there is too much CPU load or something? Is there a RAM PREVIEW setting or something that says if its gonna be hokey don't preview it?
 
Odd but I was able to get my master export , good thing this project is almost done.

Try raising the MIDI Prepare Using buffer to 500ms if it's at the default of 250, or higher if its already at 500. The default was lowered from 500 to 250 with the release of X1, and a lot of systems/projects don't work well at that lower setting.