I'm encountering the same issue.
On a horizontal sprite which has a 45° angle (purple), the player can walk up.
On a slope which was drawn that way (blue), the player can't.
What is the best way to make the player properly walk up on the blue ones ?
edit : okay my bad, the collision box were messed up on the blue ones. I had to remove some points and now it works perfectly. Also, it was not the same issue as roracle.