Took the amp out of storage where it had been for a couple of years.
Tubes seem to be okay, both glow and voltage is stable.
Right MOSFET channel measures 0 mV no matter what - turn the trimpot all the way clockwise, then 10-20 turns counterclockwise, nothing, although sometimes the mV will fluctuate between 200mV - 1V, but move the trimpot anymore and it goes ack to 0.
Left MOSFET channel is stable and can be adjusted by the trimpot.
I switched the tubes to make sure it wasn't the tubes that was causing the problem.