Why does the belt on a riccar vacuum keeps coming off?

The cause could be different depending on which model you have. Some models have stretch belts which should be replaced at least once per year because they lose elasticity. The most common problem I've found with Riccar vacuums is the belt coming off when a new belt is installed on an older model. Riccar vacuums are very well built and many owners use their vacuum for many years without having it serviced. I recently had a 17 year old Riccar Vibrance model brought in for service because the belt crumbled to pieces. The vacuum had the original belt and filters. Putting a tight new belt on the vacuum pulls on the motor because it must be stretched so much. After many hours and years of use screws may need tightened up and motor mounts and pivot points can become worn.
If your model is a few years old or is used heavily I suggest having it inspected by a qualified Riccar service center. A properly maintained Riccar vacuum can last 10 or even 20 years. A few dollars spent on maintaining it can add years to the usable life.