the armature inside or the brushes are burn if it's not under warranty take it apart clean the armature with denatured alcohol do not use sandpaper

take the brushes out and very lightly polish them with 2500 grit metal sandpaper
then use a very slight amount of di-electric grease where the brushes contact the armature

this is how i rebuild electric motors for hydraulic pumps if i don't have parts readily available you may be able to go to radioshack and buy new brushes but i am unsure if they carry them