If I load a DX7 preset, there is no sound at first.
I have to go to the voices menu and turn them from 1 to 2 (or higher) to hear something.
If I then turn it back to 1, there now will be sound (1 voice of course then).
That's weird.
How many voices do you have for the other instruments ? Are they at max ?
Does it help if you decrease the number of voices for other instrument ?
But I cannot get polyphony on the DX7 sounds if I turn up the voice number. It will be just monophonic all the time.
Good catch, there's a problem with the DX7 sysex importer.
It sets the number of voice to 1 in the mixer and the sound type to MONO in the Editors (Edit->FM->Voice).
Change the later to poly to enable polyphony.
I'll take a look to have this done automatically.