I keep routing a MIDI track to DimPro - it works at first, but then it just goes quiet - not playing any of the MIDI sound being fed into it.
Any thoughts?

Same happens to me...only thing I can do is insert another instance of dim pro and hope it works, or restart Sonar 

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

The best way to determine if it's Dim Pro (or any other instrument), or the MIDI data that you are trying to play back is causing the issue, is to create a new test project with just Dim Pro inserted.
 
Open up the Piano Roll View and insert a few notes.  Does Dim Pro play back the notes you have input?
 
If so, then scook's advice to edit your original MIDI file to remove the problematic MIDI data will help.

Make sure it hasn't switched to demo mode.

Also make sure your MIDI project is not sending NPRN or RPN events to Dimension pro.  Doing so will stop Dimension pro in its tracks.  To test this you can configure the synth to not intercept NPRN events.

1.  Use event list view to see if there are any stray controller messages being sent to DimPro (including NPRN or RPN events, as promidi says).
 
2.  Make sure "zero all controllers" is checked in settings.
 
my 2 cents