This could be a bug. I have had this happen with SI instruments. I rarely use them but I have seen this. What seems to work is starting a new project and either importing what was in the old or copying and pasting.

Save your project with another name. Such as Project A or B etc.
 
Select all the tracks with the problem, Open the Even list. make sure that the RPNs and NRPNs are the only things visible. Delete the RPNs and NRPNs, save your project, close Sonar and reopen it.
 
HTH

Okay, you probably won't believe this, but I have not touched SONAR since I posted last Sunday, I swear, and you may have guessed, IT WORKS!  I swear, I did nothing.  I went back into the song which was left with nothing in the synth rack last time I used it, I added Dimension Pro, loaded a bass, went to the MIDI bass track set the output to Dimension Pro, and it freaking works...
 
I don't know if I'm happy or pissed off... either at not learning what the problem was or that I was just spacing out... No, I've had this program since version 4, I know it.  And no, I don't get high!  Ha ha ha...

I never found out what caused the problem either. 

Oh, and thanks to every one that helped.  It's nice knowing you're out there.

Glad it's working.  I had a stereo bus effect stop working all of a sudden.  Stopped Sonar, restarted, then all was well.  Been fine since then.

We knew it would work if you found the magic incantation. :)