troy-bilt pony, blades slow down when deck is lowered ????

The tractor runs fine but when you engage the pto blades spin as normal but when you start to lower the deck each notch the blades slow down, at the lowest level the blades turn but very very slow won't gut the grass. HELP PLEASE

It sounds like the belt isn't staying tight as you lower the deck. Without knowing your model number I am only guessing. If this engages the deck as you lower it, the deck may be moving forward loosening the belt. If you have a separate lever to engage the blades, then check the idler pulley system for free movement, and that it is working correctly.