Swap J13 with J12. If the problem was with the bottom shelf, did the problem remain at bottom shelf? If yes, either missing +12 at motor, harnessing from J13, pin 2 to ground side of motor is bad, or bad motor/gearbox assembly. If no, then something is wrong with the main board.
If the problem was with the top shelf, if the problem remains at the top shelf, then it is either missing +12 at the motor, harnessing from J12, pin 2 to the ground side of the motor is bad, or bad motor/gearbox assembly. If no, then something is wrong with the main board.
If you need further help isolating the wiring fault refer to our system schematic from the article below: Snow Ball toss - manual