There are a few things that might be the culprit on this one. Egg whites need to be beaten longer: make sure the egg whites have stiff peaks. If they are not, keep beating them. They should be stiff and glossy when you are done with them.  Mixing too much: Overmixing the batter will result in deflated egg whites, and the final dessert will not have layers. Oven temperature: this is a big problem with many ovens. I suggest an oven thermometer so you know exactly what temperature you have inside. Overbaking the batter might be another issue.