Greetings from GoVacuum.com,
Have you replaced the belt with a new one yet? You may want to try replacing the belt, as with flat belts like the Fantom Thunder, they do stretch out 4 -5 inches after time. We in fact recommended replacing a flat belt every year, even if the belt doesn't break, because of the stretching. I think what may have happened, is that your belt is stretched to the point where it will not stay on the pulley as soon as there is any tension (putting the brushes on carpeting), which will cause the belt just to slip off.
You should be able to replace the belt (very inexpensive, we sell Fantom belt for $2) and this should take care of the problem. When you replace the belt, you should also take the roller brush apart and remove any hair, or thread that may be wrapped around the bearings. You should be able to remove the bearing cap by pulling them, or holding one end, and unscrewing the other.
I'd even be willing to send you a free Fantom Thunder belt, if you'd be so kind to vote Fixya for this answer :-) Just send me an email with your name, address, and the exact model number (located on the back of the machine, or the bottom of it) of your vacuum, and vote Fixya, I'll immediately ship you the belt for free!
Please let me know if you have any other questions I can help you with.