dirt devils have bronze-bearings instead of roller bearings( better) If they get gummed up or ; as the gentleman above answer suggested, bound up with string and carpet fiber's, they will burn off a belt in no time. You must take the roller assem. apart and clean and oil it. NO 4in one oil ( kerosine base) its bad for bronze bearings) turbine oil is best ( hardware store)
Clean and re-assemble. Watch out for excess oil on belt or wheels!