If your angel food cake is not fluffy, it can be for three main reasons. First, if you overwhip the egg whites, you can whip out the air bubbles. The second cause is if you use container egg whites instead of breaking individual eggs and separating the whites. Those containers of egg whites are often pre-frozen which can affect how the eggs behave. Lastly, if you overmix the cake batter when mixing in the gluten free flour, it will pop the air bubbles, making your cake denser.