- Hollow shells can be from egg whites that are not aged, therefore the protein structure is not strong enough to make a proper shell. Age your egg whites for at least 48 hours or be sure to add the meringue powder to strengthen your shells. - Another cause could be trapped air before baking. Be sure you bang your tray a couple of times to release any trapped bubbles in the macarons before baking. - The oven temperature could be too hot. Decrease the temperature by 25 degrees the next time you bake. - Improper mixing. Make sure you are mixing your batter to the correct consistency