Why do my BRAND NEW vacuum cleaners always stop working right after only a few weeks?

I don't misuse them. I simply turn them on, vacuum the carpeted floors in my apartment, and promptly turn it off and store it in the laundry room. I have now had to purchase 3 vacuum cleaners in a year because they all have the same problem. They just stop picking things up whatsoever; however if you use the attached hose it sucks everything up. There are no clogs and nothing is wrong on the bottom in appearance. Help!