This is likely due to two missteps: 1. You over mixed the batter. Over mixing develops the gluten strands in the flour, creating a more dense and heavy batter. Mix just until the ingredients are combined. This is why it's important to mix the batter by hand. 2. Your pan is too small. 9x5 loaf pans can vary in size, as mine is about 9.5x5x5 inches. If you have a smaller loaf pan, only fill the pan about 2/3 full. Any more than than and the center of the banana bread will have trouble baking all the way through.