how can I fix my faulty ford focus central locking?

I have a remote key fob, but the door lock button has always been damaged, so it will lock the doors remotely but I have to insert the key in the door to unlock the central locking. My problem is that I can unlock the drivers door, but when it`s cold the other three doors don`t unlock immediately and can take several minutes to do so. Also, sometimes, when locking the central locking, the locking sound is very weak. Some time ago I changed the door lock latch in the drivers door with a second hand one which seemed to fix the problem for a while, but the fault has re-appeared