Logic Step Sequencer losing sound output

Got a weird one. When I create a new Software Instrument track and open up the step sequencer and try to create a new pattern, sometimes - often - I get no sound when I turn on the preview and add notes. other times, some instruments work, others do not. and just now, as I was typing this post and had the sequencer looping, it just stopped outputting sound. If I play the actual track, I get the sound back and then I can hear it from the preview again, but it's very intermittent.

does anyone know what is up with this?