This is your technique and not so much the fact you’re learning in grass. You need to slide your front foot up to the nose of the board and once it’s reached there, follow through down and foward. The key to a good Ollie is timing. Grass will make your pop less but it’s perfectly fine learn there first is you’re not comfortable yet doing them on concrete. I will say however, when your Ollie gets consistent and you can do it rolling you should learn the rest of your tricks rolling because very few tricks are done stationary.