Be sure to test the motor also. I had the same problem and the issue was a broken winding in the motor. Rudy's jaw would work in one direction, but not the other.

What I found in test (with the motor free of the gears) was that, when current was applied, the motor would sometime start and sometimes not, depending on the position of the rotor. Not all armatures were conducting so the motor would only start up with the armatures on the rotor in a certain position. Once rotation started, momentum allowed the motor to keep spinning. However, if the rotor wasn't in the right position, rotation wouldn't start at all.

When the motor is in the game, the rotor is locked in gear to the jaw, so the rotor is always in one position when the jaw is fully open, and in one position when the jaw is fully closed. In my situation, the motor could start if Rudy's jaw was open, but it couldn't start if his jaw was closed, due to the broken winding.

I replaced the motor, and it fixed the issue. The replacement motor is ridiculously expensive from online pinball shops. I bought one from Radio Shack for under $5, and swapped the gear wheel from the old to the new motor, which was a total pain. Radio Shack part number is 2730256.