Try removing the lock plates off the door frame, one at a time, ie remove top one and try locking door, if door locks this is your problem one, if not replace it and remove the next one down and try again. Repeat the process for all of them until you identify the problem one, there's about 4. Check the plate for damage, burrs or a rough surface and also check the corresponding bolt part on the door itself.
Colin1970
November 2006