GE GSD650X-68WB detergent dispenser won't release during cycle

GE Dishwasher runs fine, but for the last few months, the main detergent dispenser cup door will not open during the ‘normal wash’ or other cycles to release the detergent. We have to stop the cycle, open the door, and manually open the dispenser cup door to release the detergent.

Any ideas?

And are there any "exploded view" of the dishawsher and parts lists available online?