I've been trying to figure out what is causing this but now I need your help.
 
The problem is that some patches are silenced once I send a MIDI track to them.  Here's a detailed description of the situation.
1.  I open a multitrack MIDI file.
2.  I insert a soft synth using the TTS-1.
3.  I assign the tracks to the TTS-1.
At this point I get a feel for the arrangement and mix.  All the tracks are playing as expected.  But I want to use other synths for some of the tracks.
4.  I insert the new soft synth I was to use and select a patch.  I test the patch using it's built-in input.  The patch is playing through the Master output like the TTS-1 does.
5.  I assign a MIDI track output to the synth inserted in Step 4.
6.  I press PLAY to hear the track using the new soft synth.
HERE'S WHERE THE PROBLEM OCCURS
During this playback, the MIDI track involved shows output on it's meter but the soft synth added in Step 4 does not play.  What's more, I can't play it manually as I could in Step 4.
 
I look forward to getting your suggestions.
 

"4.  I insert the new soft synth I was to use and select a patch.  I test the patch using it's built-in input.  The patch is playing through the Master output like the TTS-1 does."
 
Some synths require the patch to be set in the MIDI track pane. Selecting a patch in the synth's browser only allows you to audition the sound.
If you haven't chosen a bank and patch in the MIDI track, try that.

There can be all kinds of controller messages in a third-party MIDI file that can cause a particular synth/patch to go silent. Try enabling 'Do Not Intercept NRPNs' in the synth properties. If that doesn't get it, check the Event List view for controllers for Volume, Expression, Modulation, Attack Time (very long attack can effectively silence short notes), etc, and try deleting them.