Unlocking Rear Door

I have a 2006 octavia pd diesel engine and have trouble with N/S rear door . When I lock and unlock the car with the key all locks work just fine , When i try to lock the doors with the rocker switch on the inside panel all the doors lock apart from the N/S rear , it locks and the instantly unlocks . I have changed the door lock assembly but this has made no difference also checked the wiring in the door all seems OK Help any ideas Thanks Chris 595959