Here are a couple of trouble shooting ideas: 1.) Baking powder loses its leavening power if it’s more than 6 months old. 2.) Over beating once you add the flour. 3.) An inaccurate oven temperature (most oven temperatures are off slightly, so it can be helpful to use an oven thermometer to check it.) 4.) If the cake batter sits out too long before you bake it, that can cause the leavening to loose its power, causing the cake to sink after you remove it from the oven.