key word:
GUMMY. If you found gummy one place you can bet there is gummy in those knobs. Like Joe said - Triflow and wait. Heat - but not melt the plastic type heat. If you try to turn the knobs when they are gummy they will turn on the shaft then you have to fix the knob and shaft problem. So don't force the knobs to turn. Getting the Triflow into the places would be the hard part. I take off cover plates, hand wheel, anything in the way of the inside of the machine. Then I oil anything that remotely might potentially ever possibly move...
Then the hard part. WAIT. Oil it again. Wait. Maybe put it near some heat. Gently try it - do over. Some people use Liquid wrench - it WILL melt any plastic parts. (ask me how I know)