It may be swagged slightly, but more likely to have carbon/sludge build up on the tube, unless it has been cleaned on the underside lately. Try soaking it for a bit with carb cleaner and see if you can rock it more. May take a healthy dose of that thing we aren't known for.....damned patience. If you happened to have a wooden dowel the right size to fit close to snug in the tube, you can use that to get more leverage to convince it to rock.....gently, not after you are thoroughly irritated...
