Shaping loaf

My first sourdough starter is doing well and I have baked 2 loaves of bread now that came out with a beautiful crust and great flavor.  I'm using a Sassafrass LaCloche to bake my bread.  However, my loaves want to spread out instead of rising into a nice boule.  I am placing the "rounded" shaped loaf on the bottom of my baker and letting it rise there for the final time.  Then, I place the LaCloche in a cold oven and allow the LaCloche to heat up as the oven heats to avoid cracking.  My first loaf rose fine and I had a nice boule.  Today, I did everything the same, but my loaf "spread" out as it rose, presenting a much "flatter" boule.  I still got nice oven spring and the crust and flavor are nice.  Any suggestions to get the nice boule without flattening out?