Door lock stuck in locked position

[Shabazmo]

My R erg 1997 polo has central locking. When I lock the car using the drivers or passengers side door key locks, all doors lock. When I try to unlock it using either door lock, all door lock poles lift up and doors unlock except the drivers side which remains locked. Using the key has no effect. When I start the car and drive off, the drivers door unlocks.

Any ideas where to start looking.
Thanks

[Shabazmo]

I think I have fixed it. I took the door panel off and squirted lots of WD40 onto the door lock mechanisam and its working ok for now. It aspears that when the door is unlocked, something in the door lock moves which allows the internal door lever and door handle to catch to open the door. The movement of the something was not happening.