How can I fix my central locking on my Mk3 Golf?

On using the key to unlock the drivers door, only the drivers door will unlock. If I then manually unlock the other 3 doors I can lock all 4 of them by using the key in the drivers door. so in summary; the central locking system will lock, but not unlock all 4 doors.

There is pneumatic noise when locking the doors but none when attempting to unlock the doors.