I would guess you underbeat the egg whites. They must do more than hold their shape, they must reach the stiff peak stage. I would also suggest you add a little cornstarch with the sugar, as you would in making a Pavlova. It is possible to overbeat egg whites, but I think you'd know before you got to that stage. 1/2 cup sugar seems like a lot for 6 egg whites, too. What size eggs?
Is your oven accurate? Do you know that it's at 200 degrees? It's worth buying an oven thermometer to make sure, nothing is more maddening than baking with an inaccurate oven.