Have an assistant help you in the following procedure. Get under the mower where you can have a good look at the tension pulley movement. Keep your hands out of the way and have your assistant move the pedal from one extreme to the other while you watch the tension pulleys move. The main drive belt is designed to be loose but the slack should be taken up as the pedal is depressed. If the old belt and new belt measure the exact same length and the slack is not being taken up when the pedal is depressed then most likely you have the belt routed incorrectly. Another possibility is that the pedal needs to be in the "up" position as you install the belt.