2.4 liter serpentine belt slipping

Hello,

After replacing the timing belt, the new serpentine belt slips off of the largest pulley, which also holds the p/s belt. It slips to the center of the engine. I have tried resetting it and increasing tension on the alternator, which holds the belt in place, but after a while it just slips again. In this engine the alternator moves front and back to create tension, btw. In case it helps, after the belt slips, the alternator has not moved. The edge of the belt has become weak and bends on the side that kept slipping, so I turned the belt around left to right, but it continues to slip. I know the belt needs to be replaced now, and I will do it, but the same thing will happen. Does anyone know what can solve this problem? Is it pulley-related? 2.4 Liter 4cyl