Originally Posted by compu_85

I'd suspect it's a torn diaphragm in one of the vacuum actuators. When the leak is small the pump's pressure output is enough to overcome it, and the door will unlock. But because of how the diaphragm flexes when locking, the vacuum leak is too great and the door won't lock.

The leak is probably in the door that doesn't work properly. Easy enough to test with a hand pump... or just start the locking cycle and listen for a hissing sound. You'll need to replace the actuator, repairs don't last long (ask me how I know...).

-J