problems with back door lock

Hi Margaret.
There are two causes that I've found with the door not catching. The first is when the door shuts very tightly and requires either a good slam or push when shutting it. It may be that if this is the cause the door was set tight to stop the wet getting in during its leak test at Romahome. The second cause is if the large moving pin on the lock is moved prior to reaching its correct position, this can be done if the pin touches the door rubber as it passes, the rubber sometimes needs to be pushed back on properly ( things move and settle shortly after use).
If you close the door fairly gently and catch it on its first click then give it a good push shut you'll hear the second click engage, that way you wont have to slam it shut.
The internals of the lock are very simple with few moving parts so lubrication of the mechanism rarely cures much. The only other possibility is the actual locking barrel may need to be lubricated just in case when you open it with the key the barrel remains in the open position which would result in the catch not able to lock in the shut position.
Thanks
Ant