Are there thumb screws on each side where the support arms attach? Loosen the thumb screws and lift up the cover from the bottom to the angle you like and re-tighten the thumb screws.
That is how the cover worked on my travel trailer...YMMV
2002 Holiday Rambler 30RKD
2000 F250 diesel extended cab short bed
2014 Demco Autoslide 18K