For fs shuvs:
Have your front foot in the middle of the board. When doing it, have the foot at an angle, like when doing flicks, or move it away from the board to give it space. Oh and you have to jump.
Alot of beginners doesnt jump, they just "jump" against the board, not giving it room to do the trick.
But yes, I cant wiff a fs shuv either, I have to kinda pop it solidly.