I've recently bought a new Ridgeback from StartCycle, and I've been having a little bit of trouble with the rear derailleur. It really doesn't like being in first gear (it refuses to go on properly and enjoys hovering above it before slipping back down to second) and it keeps clunking very loudly whenever I put it in sixth. All other gears are fine, and the cable has already been tightened and I can't tighten it any more. If I'm being a tool, please tell me but I only have one free check-up and I'd rather not waste it on something that I could sort myself.