Can you disconnect the hydraulic strut and measure the pressure when the pump is asked to lift the lid? Maybe the pump is faulty and cannot provide the pressure. I couldn't find any "official" numbers, but from various forums I can deduct that for lifting the tailgate the hydraulic pressure should be somewhere above 60 bar but not exceeding 100 bar. Below 60 bar when closing the lid and around 40 bar at all times to dampen the movement (so that the lid sags instead of dropping if the system is faulty).