Quick breads will occasionally sink in the middle after baking. This can happen for a variety of reasons, but I don’t let it bother me. We tend to like ours a little gooey in the center so I occasionally underbake it just a bit. You can try baking for longer, but it may still sink. (If you look at my photos, the one in the pan hasn’t sunk. But if you look at the others, it has sunk slightly.)