Citroen C4 front passenger door won't open

Front passenger door doesn't open every time from the outside, its been in the garage 4-5 times now and they cannot find the problem as when its with them, sods law it opens every time. They have put in a new lock mechanism and cable but its still happening. they don't think its the wires in the door crease as you can still hear the lock, locking and unlocking, when you press the key. Very frustrating for me and the garage, they suggested a new external door handle if it happens again and low and behold, this morning it wont open after being in the garage yesterday and they greased every moving part in the door.