Central Locking Problem with 1999 Ford Focus

Hi, I lock my car using key, never been a problem until today. when all doors are unlocked and i try and lock it from drivers door it turns over to lock then unlocks. If i lock it from passenger door using key all doors lock. If i manually lock passenger door and then use key in driver door - rear doors and driver door locks. If i'm sitting in car and press lock button on passenger side all doors lock, if i do it on drivers side noise shows it starts to lock then opens so wont do it on drivers side unless passenger side already locked if that is locked then it will lock other 3 doors. Hope you can help Cheers D