There may be a couple of reasons that your Japanese jiggly cheesecake is not fluffy: 1. You may have under whipped your egg whites. Make sure the egg whites are whipped until they reach medium-soft peaks. This means when you lift your whisk out of the egg whites they slowly fold over the tip. The meringue should look glossy and shiny. 2. Your egg whites are under folded in the batter. You may have under-mixed the egg whites into the batter leaving a denser bottom and an airy top. 3. Your egg whites are over-mixed in the batter. You may have over-mixed your egg whites deflating all the air in the egg whites which creates a flat and dense cake.