My MMR had the same issue when delivered. There is a switch inside the castle, easily accessible from the swinging smaller door on the left side of the drawbridge. If this switch isn't working or is bent so it does not register, then your castle won't explode. I managed to bend the switch with my fingers and it now works almost all of the time. Also, you can test the switch through the control panel and confirm if it is registering. If the switch is completely bad, then contact Planetary Pinball parts department. They can send you a replacement, which is free if you purchased an MMR.