The fan belt was loose, so I replaced it. I cleaned up the pulleys with sandpaper, and the new belt fit right with the same shims in the pulley.
I peeled back the fabric to see what the problem was with the false ribs that weren’t sitting right. They had come loose from the front spar, but were still stuck on the wooden stringer, and had twisted over. You can see where one was broken and had been repaired. The repair had failed, and you could see a point in the fabric where the break poked up.