Scrolling mobile prototypes with fixed elements

Hello, I am trying to get my prototype of a mobile app to have a fixed (not scrolling) header and footer. The elements don’t scroll when I select “fixed position” but the content behind them does not scroll all the way, leaving a bit of desired space on the page behind the fixed elements. Anyone know how I can get the page behind the fixed elements to scroll all the way? Anyone else running into this issue?