trash compactor jen air

Compactor works well except for the following: when you compact the trash the ram does not cycle back up…the motor just sits and hums until the cut out kicks in … unit sits for awhile and motor tries to finish the cycle but hums until cut out kicks in again. If you rap the outside of the compactor them the unit will finish the cycle. this happens even when the trash bin is empty. Appreciate any help that you can give. Thanks Cliff