I recorded (SONAR Platinum latest release) a MIDI trk that outputs to a K2600XS synth and plays back with no issues.  I then set up a Dim Pro trk and copy the MIDI data from the Kurzweil trk. I then solo the Dim Pro trk and hit Play and hear nothing (I do see activity in the Dim Pro record meters though). This also happens with my Rapture VST's BUT not with my Zeta VST's - they play back with with no issues. Looking at the Event List, there is a Control 7-Volume cmd at the beginning that is set to 103 (typical note volumes are in the mid 60's to low 80's). The Dim Pro trk I/O settings are the same as the Zeta.  Any feedback appreciated - thanks.
 
-Dave

I assume you've checked the routing, to make sure that the DimPro is going to a track/buss that's not muted down the line?
Whenever I have an issue like that, it's usually something THAT obvious.

The Dim Pro VST's get set up with the same routing as my Zeta VSTs (Omni input going to the VST then the VST output to the Master).
-Dave

Also check midi output channel. Probably Dim Pro needs channel 1.

http://forum.cakewalk.com/FindPost/3235786