Beater bar belt too loose

Question

My beater bar v-belt doesn't seem to get tight enough when engaged. The beater bar slows or stops when it contacts the carpet, and as a result the flat drive belt spins slower than the motor and friction causes it to overheat and smell like burning rubber. I cleaned out all obstructions so the bar spins freely when not engaged. It seems the belt should be about 1/8 " shorter than it already is. Any ideas ?