The belt looks as if it slips off, and the motor will suddenly reve down very low if it's been on for awhile. The brushes, though, don't appear to be spinning.

From the top side of the motorized nozzle, grasp the hood near the handle socket and lift the hood forward to remove. Lift the brush roll from the pulley end of the base to remove the worn belt from the brush roll. Replace the new belt around the brush roll pulley and return the brush roll into the base.
https://www.allegrovacuums.com/spare-parts/drive-belt-for-sweep-n-groom-powerhead-zas099-zas099-bt.html

1. clean the brushes of hair etc and manually turn to see if any obstructions.
2. check the belt and its tension.

It should work.

It's most likely the belt. A loose or worn belt will slip on the motor shaft and cause the burning odor. Take a look at it first, check for obvious wear and/or melting.
However, if the brush is spinning properly, it could be the motor on it's last legs.

Check the tube that leads down to the brush head for clogging as well as any motor or HEPA filters. If that still shows nothing, take the cover off the brush head (like you're changing the belt) and look for obstructions in there. Usually, if you have low suction on the accessory hose, the problem is with a motor filter, HEPA filter, or a clog where the hose goes into the unit. If you have good suction on the accessory hose but none on the floor, it's usually in the brush head (clog or broken belt) Good luck

Looking at the owner manual, there is no tensioner assembly.
If the motor is fixed in place and not loose, then you either have the wrong belt, or the brush roller is not properly installed, or the brush roller is worn out where the belt goes around the brush roller.

Even though the brush is spinning, you probably need to replace the belt because it is stretched. It is probably slipping on the beater bar...

Jim

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.

Hi, W/D here. Easy fix!
Flip the machine over, beater bar side up. The base plate has to come off; it is held on with either screws or clips or a combination of both. Once the base plate is off, you'll need to carefully remove the beater bar, paying close attention as to how the end pieces come out with the beater bar from the vacuum frame. They have to mate back up in the same way they come out. Also note where the belt will ride on the beater bar. While you have it torn down this far, clean the suction cavity as vacuums usually plug up in this area, and remove any hair, ribbons, thread and the like that has gotten wound around the beater bar. Slip your belt over the motor shaft and then slip the beater bar into the belt. It'll feel like you need another pair of hands, but it can be done. Slip the beater bar and sockets back into the vacuum frame, and look again to make sure that the ends are seated properly. Test your work by revolving the belt a bit by hand, feeling for the resistance of the belt and smooth operation. Reinstall the base plate. That ought'a 'bout do it!
Best regards, --W/D--

first check if the brush motor is turning before you spend money on a new brush roller and belt. remove the belt by unscrewing the screws holding the brush cover ( the part touching the floor ) and slip the belt of the rotor. turn the vacuum on, and the brush on while the vacuum is in a reclined position. if the motor is not moving, you need a new motor.