Replace the belt with a brand new one, they are not expensive. Old belts can get a glaze on the surface which can cause the belt to slip even when the tension is correct. A new belt should cure the problem unless you have a more serious issue.
ChicksFan, November 2012
there is nothing else you can do but remove the belt,you may have another pulley/idler at fault or you may be lucky and only need a new fan belt,good luck
