OK I figured ou how to achieve the wall dash every time (press dash and jump at the same time without A or D pressed), but I think it would be better if you could wall dash when you respect these conditions :
- touching a wall
- press DASH (it won't do anything as we are not on the ground)
- as long as you don't touch the ground, if you press JUMP (with DASH still pressed), the ball does a wall dash
(now you have to press DASH and JUMP almost at the same time, it got me a lot of trial to understand it)