Not familiar with the specific game, but it's probably a 'stepper': http://www.pinrepair.com/em/index2.htm#steps
Prop up the playfield, and start advancing the light and watch for what mechanism moves (it might also be in the backbox). It'll have two solenoids, one to step it up, and one to reset it once you score. Make sure the reset solenoid is at least trying to fire when it's time, and it's not an electrical problem causing it.