If the coffee cake is sinking in the middle, there are two reasons that are most common. The first is that the cake is underbaked. If the cake is not baked long enough, it may not set properly, causing the center to sink. Make sure to follow the recommended baking time in the recipe and perform the toothpick test to check for doneness. The second is that the pan may be too small. This coffee cake recipe makes a generous amount of batter and it bakes up TALL. We used a deep 9"x13" baking dish. If you have a dish handy that's even slightly larger, we would recommend that as well. Start checking for doneness at the 45 minute mark if you are using a larger pan.