Sinking is usually caused by one of two things. 1. Under baking the bread. You can tell when quick bread is done by using any of these methods: a) Insert a toothpick into the center and when it comes out with just a few crumbs, remove it from the oven. b) Lightly press down on the top of the loaf. It's done when it immediately springs back. c) Use an instant-read thermometer. Insert it into the center of the bread and back to 200°F. 2. Baking soda or baking powder is no longer active. To test it, see our Baking Techniques, Tools, and Ingredients post.