Kenny, there has to be some kind of belt tensioner for the drive belt that runs the blades. This will have a spring assembly of some kind that controls the belt tightness. You might check this out. Even with a new belt, the pulleys are probably very smooth and/or have a lot of build-up of grass clipping mashed into them and mud and crud. This can allow the belt to slip so being sure that the drive surfaces of the pulleys are clean is also necessary.