yeah generally what they do is put a single electric central locking motor in the driver side door, alongside the factory stuff. what I would do is try locking/unlocking the door manualy from the key/lock knob. if the same problem occurs check the switch in the door for problems. if the lock/knob work perfectly then i would have a look at the central locking motor, could be its getting a bit tired and not pulling the pin fully. they're generic units, can buy em from maplin and the like. just gotta make sure you get the right type, you get those with 2 wires (slave motor) and those with 5 wires (trigger motor)
