Maybe someone can help me - i'm going mad!

I have a problem with drivers door lock. It will lock and unlock fine with the key, and locks fine with the remote. However, I cant unlock the drivers door at all using the remote. The other 3 doors all open fine with the remote.

I've taken the actuator out (the part that contains the two electric motors), swapped to passenger door, and the actuator works fine on that side. likewise, the passenger one works on the driver door too. Unfortunately i cant keep them that way, as they are side specific. looking at the mechanicals inside the actuator there doesnt appear to be anything wrong. I'm at a loss!
Any suggestions from you guys as to what to look for next ?

thanks for any suggestions.