Replace the belt. All belts stretch with time, lose flexibility, crack and become rigid. Belts develop a "glaze" so to say where they contact the pulley's, and can start to slip requiring tightening to maintain non slipping contact, which stretches belts even further, and it's a compounding problem. Belts are expendable items that
should be replaced every so often. A new soft flexible belt can make a world of difference.