1) You forgot to add baking powder, or you used expired baking powder. 2) Your pan is too big, so the mixture can't rise enough to fill it. Or 3) You over whisked.
Problem #1: The cake is dry. Problem #2: The cake is wet on the bottom. Problem #3: The cake sunk in the middle. Problem #4: The cake is done or over-baked on the outside, but not done in the middle.
Too much leavening agent like baking soda or powder can cause a cake to rise too high too quickly. The gas from the leavening agents builds up and escapes before the cake bakes through in the center. This causes the center to collapse and makes your cake layers sink in the middle.
If your cake isn't moist enough, it can sink in the center. But too much moisture can also ruin a cake. This happens most often in humid climates, where extra moisture can collect naturally in ingredients like flour. It causes cakes to rise quickly and then crater during the baking process.
--Underbeating and overbeating eggs, particularly egg whites, can result in poor volume, dense cakes. Overbeaten egg whites are harder to incorporate and may produce a dry cake. Bring egg whites to room temperature before beating for best results.
Over mixing. Too much sugar, shortening or liquid. Too low an oven temperature. Insufficient baking.
When you overmix cake batter, the gluten in the flour can form elastic gluten strands – resulting in a more dense, chewy texture. The white batter looks airier, while the red looks thick and dense. You Can Taste The Difference: The overmixed cupcakes were gummy.
Room Temperature Butter / Don't Over-Cream
Butter is capable of holding air and the creaming process is when butter traps that air. While baking, that trapped air expands and produces a fluffy cake. No properly creamed butter = no air = no fluffiness. Aka a dense cake.
If your oven is too cool:
Your cake may have coarse, dense or heavy texture. Your cake won't set fast enough and will fall. Your crust may be too light. If your oven temperature is uneven, your cake may be uneven.
Cakes typically bake between 325 to 450 degrees F (see chart with Tip #9). Most convection ovens require lowering the temperature by 25 to 50 degrees F, as well as turning off the fan.
Cake Is Heavy or Dense
Possible reasons your cake is heavy/dense: Too little baking powder. Too many eggs. Batter is overmixed.
Bulges around cakes are usually the result of the pressure built up inside the cake from the time it's baked to the time it's decorated. The best way to take care of the pressure, is to first let your cake settle. Let it cool completely before you even start to level, crumb coat, or decorate.
The most common reasons a cake sinks in the middle include the following: The pan is too small. There's too much liquid. Opening the oven or moving pans during baking.
Breads, cakes, cookies, and nearly all baked goods require a leavening agent. These are the key ingredients that make a cake rise. There are two types of leavening agents, chemical (baking soda and baking powder) and biological (yeast).
Baking powder is mixed with the flour. When water is added to this flour to make dough, baking powder undergoes a chemical reaction during which carbon dioxide gas is produced. This carbon dioxide gas gets trapped into the dough and bubbles out which causes the cake to rise making it soft and spongy.
Overmixing is exactly what it sounds like: the process by which a dough or batter gets mixed too much, typically yielding dense, tough, or deflated baked goods. Overmixed doughs and batters may have an unappealing look or feel, which remain just as unappealing when they're baked.