Manual PTO not engaging blades

Hello - I have a White Outdoor LT-1650 tractor. When the tractor has been running for 15 mins or longer, the manual PTO will not engage the blades. I replaced the belt and spring thinking they may have become stretched out. This didn’t solve the problem. The engine pulley is spinning fine. When I turn the PTO lever, the pulley pushed out fine. It just doesn’t seem to be putting enough pressure on the belt. I can’t find any other adjustments that would solve this issue. Any help would be great.