E30 Zone

OK, when I unlock the car from the driver's door the passenger side unlocks - Great..!

BUT -

When I lock the car from the driver's door the passenger side stays unlocked - Grrrr..!

Any ideas guys..?

Check that key in the the lock barrel feels free to turn, and gives a positive movement (i.e.: not under any force). Also once inside the car see if the drivers side door pin when raised and lowered - operates the passenger side pin (i.e.: locks and unlocks).
It might be simple lubrication, or a worn lock mechanism?
One long term solution is to have fitted a remote locking alarm - then you can do away with key operation. Does the boot lock / unlock okay?
Regards,
pm9575

Yes, the boot and fuel filler flap locks and unlocks OK. When I'm in the car I can push the driver's door lock knob up and down. When I do this the passenger's door lock knob unlocks (goes up) but doesn't lock (go down).

sounds like a dodgy pass door motor