Midi sound reset?

Manual “reactivation” of MIDI instrument

I’m sure this wasn’t happening yesterday, but I don’t know what I’ve changed! When I’m playing back a midi track, everytime I restart playback or move the position marker (or whatever it’s called) I’ve got to go to Track Properties and change the Channel number, then change it back again before any sound comes out of the MIDI instrument. It seems to apply on both my keyboards, so I don’t think there’s anything wrong at the instrument end.