At this point I’d check the electronic parts that control the motor. It’s getting send 12V or -12V depending on the way it goes.
Try your motor this way:
Find a 12v adapter, remove connector.
Disconnect the motor engine from the connector.
Put (through crocodile plugs or whatever you want) the cables from your adapter to the motor.
Put back down the playfield so that it lies as when you play
If it behaves as it should (meaning going up and down as these are the ibly things you control with this test) when receiving power this way, then I’d say the issue is on the board or logic and not the motor.
If it does not then you received a faulty motor, or your 12V is screwing them up.