Hello Duncan and Welcome!
I'm Billy and i will assist you with your inquiry.
Feel free to ask if any further info is needed.
My apologies for the delay, thank you for your patience.
The most likely reason for this problem, would be either a faulty or a stuck door lock actuator. Accordingly, you need to start with the driver's door, remove the trim panel for access, and check the wiring on the door lock actuator, and spray some WD40 (or equivalent) on the actuator, make sure it is not jamming against anything, and then try to operate it. If it does not work, then remove the connector on it, and try to operate, using the remote fob, and if the passenger side works as it should, then the actuator on the driver's door needs replacing.
Here are some details that should help you tackle this problem:click here for details
Thank you for your trust! Get back to me if you need more assistance...Thank you in advance for the positive rating...