If your quick bread sinks in the middle, there may be too much liquid in proportion to the dry ingredients–make sure you measure ingredients accurately. It may also mean that your baking powder is no longer active. To test whether your baking powder is still good, combine 1 teaspoon baking powder with 1/3 cup hot water. If it bubbles, it is still good. If not, replace it. Never buy baking powder that is past the expiration date printed on the bottom of the can.