Mk 2 intermittant central locking issue

[LJMoxham]

I have a 55 plate Focus Ghia which most of the time works fine. But lately it has developed a problem where not all the doors unlock when i unlock the car. Its as if they become stuck and its always one of the same 2 doors (front passenger or rear drivers side & they seem to take it in turns!) I have tried reprogramming with no luck, any ideas or advice would be gratefully recieved, thank you.