Suddenly no sound

On Cubase 14, when Im using a MIDI instrument, whether Halion Sonic, Retrologue, Groove Agent SE, I’ll have the instrument panel open and will: turn a knob on the instrument and suddenly there is NO sound from the MIDI instrument, anymore. It’s reading the incoming MIDI note signal because I can see it on the MIDI In activity monitor in the lower right. However, when I duplicate the track/ instrument, no problems at all, until I turn a knob/ slider on the instrument, then NO sound again. Any help here? Thanks

Cubase 14.0.41
Mac Sequoia 15.6.1