Could be a few things - if the game nows a switch is not working that deals with the trunk, it will not spin it so you can just collect jackpots by hitting the side. You need to lock three balls to start multiball - the last one being the side of the trunk the other two behind the trunk into the trap door.
I would check
1. opto interrupter - black disc that spins and tells trunk where to stop for each side
2. all switches in subway and trap door area
3. Eddy board in front of the trunk