check belt tension,it maybe too slack and simply slipping.as you didn't state make or model,iam going to suggest inspecting the carbon motor brushes for wear.that's if yours have them?should be no shorter than 10mm.some models need to have their belts fitted the right way around ,if your belt hasn't got a toothed side then you are ok.did the old belt snap?and have you checked the drum bearings?.
mech monster, November 2011
