I am working on a Trek 800 21 speed. I am having an issue with the rear derailleur. It won't shift down to 7th gear. If I manually move the chain to 7 it stays there, and shifts back up normally. But when I try to shift back down it immediately refuses to go into 2nd until it is shifted to 3rd. I can move it back up to second and it will stay in 2nd gear but repeats the refusal to move again.