1988 Mazda 323 Non-remote central locking failed on one doo

3 of four doors lock and unlock when using key in drivers door.

I cannot however lock the front passehger door. If I phsically lock the door from the inside, the button moves but the door is still open.

Cannot lock it from inside or out, so the car is insecure all the time.

On inspection the inside door lock does move when locking/unlocking the driver door, just not enough.

I don't care if I can't open it, just need to get it locked.

Any help appreciated.

If you can be as basic as possible, would appreciate it - Can follow instructions but please don't assume any knowledge or level of expertise!

Thanks