sounds like a possible defective vacum actuator,or possible leak,in the central locking system pump,this will cause an issue with unlocking the rear hatch.did you lube up the locking mechanism with some good white teflon enhanced lithium grease,maybe its just sticking,also check for proper alingment bewteen the locking clasp,and the hatch locking piece that is grasped from the locking reciever.