Any sourdough pros around? I'm having a problem.

I am trying to add sourdough bread to the varieties I bake.

I believe my starter is good (very bubbly and I pay attention to it.) But I am having problems in the next 2 stages.

In the 2 recipes I have tried, the first step is to create a "sponge" using the starter. Once the sponge is mixed, it should be placed in a warm place to proof for 8-12 hours. In the next step the remaining ingredients are added and the mixture is left to rise for another period. In both recipes, it is said that I should see significant rising. Enough to be noticeable.

My problem: my sponge does not increase in bulk in any perceptible manner. And therefore neither does my mixture in step 2. I did try to bake it both times to see if it would rise it remained too dense even though the flavor was good.

Any ideas on what I may be doing wrong.

Thanks

BB