Lazy central locking to Ford Mondeo MK2?

My Ford mondeo ghia estate mk2 has what I would call lazy central locking, I use the key in the lock drivers side and the crentral locking works all locks,. If I then unlock the car immediately after locking it all the central locking works. However if I leave the car for a while when I return the key operates the drivers lock but the other locks do not operate for anything up to a minute or two later, wherein they then all open. Anyone offer advice on the cause of this?

Thanks

Bob