You could do with having a check behind a door card to see if you are still using the original VW air powered central locking or if someones replaced it with an aftermarket electric one.
If its a VW one then its the pump or an air leak from one of the pipes.
I'm guessing if the lockings triggered from the drivers door then there must be something there that senses the locking and tells the rest to lock. Could be that as well?