The central locking is playing up a bit, and I would like to trace whether it is the door lock/solenoid itself, or perhaps the control system at fault.
I have the car set to lock all doors once the road speed reaches 4MPH, so bearing that in mind, here is the pattern of behaviour:
It will always lock OK when I use the plip to lock the whole car when parked.
It will never lock when called upon by the car reaching 4MPH (all other doors do lock)
It will never unlock whether from the plip, or if I manuallu lock it while in the vehicle, and then turn off ignition once parked (at which time all other doors do unlock)
If it never locked at all I would go straight to the door itself to check, but the fact that the plip will lock it throws me a little!