There are a few common reasons that sourdough might not rise in the oven. 1. Your starter is too young/weak. This is especially common if you've just recently made your own sourdough starter. Unfortunately it can take weeks or months for a new starter to be strong enough to bake with. 2. The dough is over proofed. This is most likely to happen during bulk fermentation as opposed to the cold proof in the fridge. If your dough ferments for too long, all the gluten will be broken down and consumed by the yeast before you bake yielding a flat loaf. 3. The dutch oven is too hot. While you want to preheat your dutch oven for about 20 minutes before baking, you don't want it to get too hot or else the outside of the bread will set too quickly and prevent the bread from rising.