How can I fix my toyota central locking problem?

I can't unlock the driver's door using my key. It turns a bit but not fully. Locking the doors using driver's door lock. I don't have problem locking and unlocking all doors using passenger door lock or lock/unlock button inside the car.

My car is toyota camry, CSI, 1993 model.
I would like some help about how to find the problem and generally how the whole central locking system works.

Thankyou very much