Matrix view won't play MIDI synths

I'm assuming you do, but just in case, do you have synth setup to output to?

I said in my original post that there are audio tracks assigned and that they play fine in the track view.
 
Turns out no MIDI clips play unless I click the 'D' button to set the row to 'Direct' - but the MIDI tracks in question have no processing to be bypassed! This also doesn't work for my drum track which uses a drum map - there is no 'D' button there to be clicked.
 
Any ideas?

Hi  Ben,
 
I hope you don't mind a couple of more questions, but are the MIDI loops themselves, that is the notes themselves, are in the proper range (pitch if you will) to line up with your drum map?  Further, is the MV row where the loop(s) is inserted routed to the desired MIDI track? 
 
Ii have used the Matrix View quite often with expected results, including drum mapping, etc.  I have had to rearrange the notes in the PRV to match things up properly, however.
 
Hit back and I will duplicate your steps myself for further trouble shooting.
 
Kind regards,
 
Living Room Rocker

Are your MIDI loops routed through the drum map when inserted into the TV?