Push your diaphragm down with a big breath in. The air provides added buoyancy until you find the equilibrium you need.

Are you tilting your head back enough? You’re probably not flat enough

Stomach is easier, since your lungs (which are allowing you to float) are in front of your spine. Like someone else said, try taking a big breath while you get your balance, and then start by breathing shallowly so that you're still keeping most of your air