Not a new problem. I have this happen on an Eureka vacuum that I inherited from my Grandmother in 1974. It’s probably over a half-century old now. Still works good, especially at picking up cat hair.
- replace the belt. They can get ‘glazed’ in use, and thus much more likely to slip off. Rub it, does it feel ‘rubbery’ or are there slick spots on it. Belts are cheap enough to try this.
- check around the axle on the roller/brushes of it. Pet hair, carpet threads, etc. tend to get wound up around there. This resistance seems to make the belt more likely to jump off. Also check the hub that the belt rides on. That can get dirty or polished slick by the belt, and then it slips off easier.
- Check: does this model have a height adjustment? If you have it set for bare floors and use it on carpets, the increased resistance for the roller makes the belt more likely to pop off.
- silly question, but are you sure you’re putting the belt back on in the correct direction? It’s easy to do this if you aren’t paying attention, or haven’t done if for a while. If you do it wrong, the brush rotates fine, but in the backwards direction, and will pop off easily.
Finally, just live with it. My antique one does a real good job, better on pet hair than many newer, more expensive* vacuums, so I live with this occasional problem.
*Grandma was moving back to the country, and selling off much of her stuff. I paid $24 for this vacuum. She said $25 was too much to charge for a vacuum with many years of use already, so that was all she would take. A cost of 60¢ per year of use so far, and still going strong. Quite the bargain.