Dough not rising enough

Hi

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?

Thanks

Rob