Belt Keeps Coming Off

\015 Belt keeps coming of motor shaft when shutting down or pressure is applied on roller in low carpet setting while operating. I placed belt back on and all seems to be in line but when I close it up and start operating it it works until I set it on low carpet or shut it off and then turn back on…any solution would be apprecitaed.\015 – Hoover U6616-900 WindTunnel Bagless Upright Vacuum