When I try the "Butt only to wall" handstand I notice severe problems. There seems to be an impossibility when I try to arch my back, open my shoulders and lean my butt (and legs) agains the wall. My back muscles get really tight and the back doesn't want to arch. This has been a great puzzle for me since I don't know what the main issue is (I know "shoulder" flexibility plays an important role but there is more to this and I can feel it). I included a video. Help is very much appreciated!