I am not new to baking but have only recently started to bake sourdough. After one or two false starts I am baking good bread, but it is dense and flat, not the tall, airy, large crumb loaves I aspire to. I have tried increasing the proving time, making the sponge or dough wetter or drier, but nothing seems to work. I am now using tins to give the loaf support, and it is helping, but I want to be able to bake large, round balls. Can anyone help?