If you, like me, are having trouble with flat sourdough, I have the cure. Get some "vital wheat gluten" and put in a tablespoon in each loaf of dough. You'll get plenty of rise and oven spring.
You're probably not getting the dough's natural gluten developed enough. But, until you've cracked the secret of getting it developed exactly right, the Vital Wheat Gluten works. Yes, it's cheating -- but only a little.