The problem is likely to be the white plastic belt drum on the clutch mechanism. As the brush bar becomes blocked or stiff, the clutch drum heats up and eventually melts. When you stop the machine, the belt tension compresses the softened plastic and you get a flat spot on the drum. The only solution is to replace the clutch itself. This involves unscrewing the 3 torx screws on the clutch cover, pulling off the adjustable head, which must be carried out with the head in the central position otherwise you will break the hinges. After detaching the hose and with the head in the normal locked upright position, you will now be able to remove the clutch cover. The large plastic button is located in two places and needs to be carefully prized off after this, you can now remove the clutch. The easiest way is to grab the belts and pull it off in one go. You then need to obtain a replacement clutch.
Gordon Smith, September 2006