Be sure you have the correct belt. Can you read the number on the belt? Also, the spring in your top picture does not have much tension. When you move the lever forward and back, the spring should be moving the bar at the back and that bar moves the pulleys to make the belt tight. Does that bar move at all? It should move a few inches. Also at the very back of the frame is a large bolt which can also move the pulleys forward and back. You may need to adjust that bolt.