First, whether the egg whites are over-beaten. Usually use an electric whisk to whip for 4 to 5 minutes. It will take longer if whipped by hand. But if you overbeat, the cake will be hard. To determine this, dip something into the egg whites and a small hammerhead will stand up. If you overbeat it, it will be hard. Also is the amount of cream of tartar and baking powder the right amount, usually two grams of each for an 8 inch cake. Finally, the temperature and time of the oven. Generally the temperature should not be too high, 140~150 degrees can be, bake on 40~45 minutes. Too long, the cake will collapse.