Have exactly the same problem this week.
Called original installer to check it out and the locking mechanism has broken off inside and needs to be replaced but the doors are around 18 years old and they may not be able to get the parts so may have to get a new door!
The only trouble we had previously was that sometimes the door wouldn't shut and the locks would catch against the plate and my mother (who didn't realise) slammed it hard resulting in it jamming.
Lynn
April 2009