Does it have the correct belt? Does the tensioner have sufficient tension? There should be 3 indicator marks on it. One on the spring side and two on the fixed side. With the belt installed the single index mark should line up between the 2 opposite from it. If not replace the tensioner. Remove and inspect the belt. Look for cracks or torn ribs. Check all pulleys by hand, spinning them and twisting them sideways. Look for bearing roughness when spinning and excessive play sideways. Any roughness or excessive play components need replacing. Check all pulleys the belt goes around. See if the belt tensioner and idler pulleys are cocked. Use a straight edge to see if all pulleys are coplanar. That is they run in line with each other. HTH