Hello, If you know for a fact that you used the right belt from the manual and put it on correctly, unclogged anything caught in the rollers, and the back isn't full and the suction is good then it sounds like the problem is with the tension on the belt, or the motor. It sounds like as soon as the brushes touch the carpet and get some actual resistance they want to stop. This sounds like an issue with the motor either being too weak or not driving the belt in order to turn the brushes. Here is a link that you might find helpful just to see what I am refering too: my vacuum roller stops spinning when i adjust the height - Google Search
