Belt won't stay on

A belt will slip off because it is stretched, or the brush bearings are worn, or the motor drive shaft is misaligned.
Hoover belts fit snug.
After as little as week they begin to stretch. Even though they are not broken, they will spin loose. IF you can stop the brush with little effort and the belt keeps moving the belt is stretched. Do not buy a belt buy size, buy it by model. But 2 and store your spare under the hood of the vac if possible.For best cleaning results,change every 3-4 months if you vacuum weekly. 6 months at most.
Most times your old belt will be stretched as much as 30% oversize. If it looks shiny inside it has likely been slipping.
Before reinstalling check that your brush rolls freely and clean the bearings at the end of the brush roll.. Once reinstalled check it again before turning on the vac.