EDIT: buy a set of feeler gauges if you don't have them. Also the idle roller should spin very freely. If not you may have dust built up. I blow mine out with the air compressor immediately after every use and tons of stuff comes out.
You added your edit after my prior reply. I have feeler gauges, but my idle roller does not spin freely. I finished off a can of compressed air on it and some things came out. It will spin for part of a turn and then catch slightly. What else should I be doing here?
EDIT - Never mind. Google really is my friend. I am going to take it apart and use wire brushes to clean the rollers (which look fairly OK to me) and the brass bushings. I will add a drop of veg oil to the bushings and follow the maker's advice and rotate the passive roller 180 degrees when I put it all back together. Pictorials make it all look very easy, but with my mad skills, something will doubtless take longer than it should.