It should have a idler arm that has pressure on it that moves in and out,either it is not in the arm pulley or the belt is stretched.
How Do Do You Tighten A Belt On A Riding Lawn Mower Craft Mans II ?
You don't buy belts from the auto parts store. Sears is bad for this 1/2 sizing the belts... You got ta go to mower shop or sears for the popper belt