I had this symptom and I got inside my door panel to find the L bracket. It turned out that in my case, the problem was with the small gray stopper cap that hits the L bracket. This cap had cracked in half for me. I tried just gluing it back together, but that immediately cracked upon my first attempt at closing the window. I then re-glued and zip tied it. That did the trick, and it seems to have worked like a charm.
Below is a pic of what the zip tied stopper cap looked like with two un-cut zip ties. I put a third on there, trimmed the tails, and encased it with glue before installing. Imgur