It seems like your motor is stopping very fast, not allowing the cams to align correctly for control of the lift down signal. Your turning the knob a little kind of proves this, it hasn't quite rotated enough to allow the signals to properly flow through the cam. Does the bad motor turn a little slower, or make more noise than the other one? Try backprobing the motor voltage at the connector without disconnecting the motor, then try it disconnected. Maybe lower voltage getting to the motor causes it to rotate slower.
Have you swapped motors and did the problem follow the motor? If so, then the motor is at fault. You might try a thin lubricating oil on the bushings of the motor shaft, that may help it rotate better.
Also, check the lift assemble for ease of lifting, maybe oil the hinge on the lift assembly, that may be binding a bit.
I've got a pile of good motors if it comes to that, just pay for postage and one is yours.
blueamber in Florida