The "door lock motor" or latch (DC's name for it) is prone to fail. They are about $85 at the dealer. Dorman/motormite and maybe Standard Motor products might offer a listing soon. I bet they are $60 or so.
Suprisingly, I've never had one fail again once the oem unit was replaced.
Assure that the door panel (if prevously removed) is not binding against the manual latch (to lock / unlock the door).
The driver's door usually gets the most use, especially with keyless entry.
Good luck
Andy