The two main issues I'm seeing are that (1) the dough hasn't risen enough before baking and (2) the bake is not hot and/or long enough. How much is your starter rising before you use it, and how long does it take to get there? How much does the dough expand before you shape it into a loaf?
The two main issues I'm seeing are that (1) the dough hasn't risen enough before baking and (2) the bake is not hot and/or long enough. How much is your starter rising before you use it, and how long does it take to get there? How much does the dough expand before you shape it into a loaf?