I think that if you look in the instructions it tells you how to make the head higher by gathering more material from the head end into the bit that you tie.
You don't put the baby at the end like you do in a cot, you put them in the middle or if you want the head up a bit, put his head up closer to the head end. Its not a flat surface so they don't wiggle up or down so much
If you email the guy who makes them he usually answers quite quickly.