Though there are a few possible reasons, a baked sponge that sinks in the centre after it's out of the oven is usually due to: - Insufficient baking time: Because ovens behave differently, the baking times are suggested guidelines but you may need slightly less or more time. Always test the cake for doneness and bake for a few minutes longer if needed. - Wrong type of flour used: Flour provides the structure in a cake as its proteins interact with fluids in the batter and affects the strength of the batter. However, if the flour’s protein content is too low, or if there is not enough flour in the recipe, the cake will lack structural integrity and may collapse.