The brief
The Jetta had one door that wasn't locking or unlocking with the central locking, the others working fine. When it's just one door playing up like that, the culprit is almost always the lock actuator on that door. He brought it in. The lock actuator is the motorised latch assembly inside the door that does the locking, unlocking and catching, driven electrically off the central locking. The little motor wears, the plastic gears strip, and the micro-switches that tell the car the door's state get flaky, so it stops working reliably. A lock actuator that's started failing only gets worse, until that door won't lock or won't unlock at all, so it needs replacing.
The diagnosis
The door card came off and the lock told the story, the actuator's motor and gears worn, not driving the latch cleanly, which is why that door wouldn't lock and unlock with the others. The wiring and the central locking module checked out, it was the lock unit on that door. That's a replacement, you don't rebuild the actuator on the car, so the call was a complete lock unit, coded and tested with the central locking.
The work
The door card was removed, the failed lock actuator unbolted and taken out, and a new genuine VW-spec lock assembly fitted, connected up, and the latch and linkages adjusted so the door catches and releases cleanly. The new unit was registered as needed so the central locking recognised it, and the door card refitted. A check confirmed the door locked and unlocked reliably from the remote and the switch along with the others, the latch catching first time and releasing cleanly.
The outcome
A door that locks and unlocks every time with the rest of the car, latches first time, and does it quietly. The Jetta went home with the lock sorted. A failing door lock only gets more unreliable until the door is stuck for good, so changing the actuator put it right before it became a being-locked-out problem.