Check the hinge brackets that hold the tailgate. If the tailgate has shifted relative to its original position, you should see some evidence of that,like slide marks or so. This would be caused by loose screws, and you just need to readjust and tighten down again.
If that's not the case, check the stops or if something is blocking the tailgate. Otherwise, it could be a problem wit the strut or hydraulics.
Rear Tailgate Adjustment Screws - How to Adjust? - Mercedes-Benz Forum
https://m.youtube.com/watch?v=F6dTrnslLSA