The correct belt is a 38528035. If the belt does not have this number printed on it, it is a generic belt and probably the wrong one. The pulley on the transmission is "convex" because this pulley design keeps the belt running in the middle. (a belt will always try to "climb up a hill") If the motor shaft and the pulley are not aligned properly, the belt will constantly throw off. This can be a result of a bad motor mount (located inside the motor compartment.) The motor shaft end has a rubber cover that absorbs vibration and keeps the motor positioned squarely. The handle could also be loose on the pivot. It is held down by two metal clamps that screw securely to the bottom, holding the handle on to the base and allowing for the handle to pivot backwards. This problem can be difficult to diagnose and I have had many in my shop for repair. It takes a seasoned and experienced repairman to find a solution to this problem, in most cases. The disadvantage I have here is that I cannot see your machines to inspect all the various things that effect the belt alignment. I hope that some of this information will help.
The adjustment knob on the hood of the machine is a plastic "arm" that prevents the belt tensioner from tightening up the v-belt so that the roller will stop or start. If the knob is adjusted while the handle is back, it can break the plastic arm off and then the function will not work. This knob should only be adjusted with the handle in the upright position. Usually, if the arm is broken, the roller will still turn when the handle is lowered and will not turn off when the knob is selected to the bare floor. If the brush roll will not switch on , it is usually a problem with the belt(s). You should replace the belts yearly. They can look fine, but be worn out.
There is an idler pulley that takes up the slack in the V belt when you pull the handle back to vacuum. Make sure it is engaging properly and the V belt is installed correctly.
Unfortunately you machine is a re manufactured vacuum and has a 6 month warranty. (The RM suffix indicates this ) You should take it to your local vac shop and let them look at it. It's probably a minor issue.
See if you can pick up a nylock nut for the pulley. As long as your nut is a standard thread and not a reverse thread fastener. These nuts are self locking. This product Devcon Plastic Welder is the best plastic repair material I've ever found. Cures in minutes,can be drilled and sanded in hours. Great stuff to fix the hole in the plastic cover.
I'm not sure what you mean by "Common Pulley". There is a flat belt that goes from the motor shaft to the big round pulley on the transmission. There is also a "V" belt that goes from the transmission to the brush roll. You have to be careful how you install the "V" belt because it has a pulley that takes the slack out of the belt when you pull the handle back if you have the lever in "Carpet" position. The belt does not get tight until you pull the handle back which leads many people to believe the belt is too long or the pulley is not working properly.
The belts may look fine but may be stretched and worn. Replace them. Make sure brush spins free. Idler pulley should tighten belt tension when handle is released
Hello! We recommend taking your vacuum into an authorized service center to have your on/off switch repaired or replaced. Follow the link to locate the nearest service center to you. Service Center Locator
Thanks!
Independent sewing and vacuum, or sew & vac stores, typically carry a wide selection of belts. Be sure to know your model number of your vacuum, even if you have the old belt, as they stretch considerably and can be difficult to match an old belt with a new one.
Check all the pipes, there could be something stuck in them which hasn't come out.
Also check the base of the vacuum if there is something stuck in the base.
Lastly check if there are any leaks in the pipes.
Also check the suction without using the handle, by pulling the pipe in your hand and see if it has more suction.
These nuts are self locking. 
This product Devcon Plastic Welder is the best plastic repair material I've ever found. Cures in minutes,can be drilled and sanded in hours. Great stuff to fix the hole in the plastic cover.
