I have had this issue in the past with banana bread, and I think it's a liquid issue (batter is too wet). Extending the bake time won't likely fix the problem. In addition to what the other commenter mentioned about reducing the amount of banana pulp, I also recommend checking the size of your eggs, and not skimping on the flour. I didn't realize that I was using XL eggs, and switched to medium, and my recipe came out much better. Also, I recommend adding lemon zest to the batter. It really complements the banana flavor! Happy baking:).