So I tried to make a sourdough loaf following the river cottage instructions on their website. The starter is great - lively and smells good. The sponge also did well but when I made the dough it rose well on the first prove but the second just didnt happen. I left it for about 6 hours but nothing. It didnt rise in the oven either and ended up being quite flat and solid with cracks on the base. Its also very sour tasting. Any ideas what I could have done wrong?