My rear derailler won't shift on to the smallest cog. Shifting is otherwise fine, but it just stops before that last gear. It's not hitting the limit screw, and the cable goes a bit slack when I put the shifter in that position. On the stand, I can push the derailler with my finger and it will shift.
This is with a new chain and cassette (though I had the same problem with an old chain and cassette), and relatively new cables/housing. The bike is a tandem, so the cable run is very long and there might be higher than usual resistance.
The derailler is an old Suntour model, and the shifter is an ancient barcon which is friction mode only.
My guess right now is that the derailler itself is sticking (perhaps the spring is worn out or the internals are gummed up with dirt). But before I try to take it apart and clean it, is there anything simpler that I might be missing?