I had a problem like this on my other E30. One of the posts that you mount the wiper arm on was stripped pretty badly. I guess it wasnt tight and wasnt seated correctly on the notched post. Try loosening the nut, set the wiper arm where you want it in the down position, then use a washer and tighten the nut, that worked good enough for me and the repair didnt let me down. The wiper arm itself, my be stripped, in either case, try to re-notch it by using a pencil file. or Dremel tool.