Central Locking on a 1989 MkII Mondeo?

For some reason, my remote locking on my Mondeo has stoped working, I have replaced the batteries in the key fob, but this has done nothing. I can lock the drivers door with the key, but nothing else will lock, I can lock the passenger door, and this will lock the other doors (sometimes). The standard alarm will activate when I lock either door, it is just the central locking seems to be playing up.