Marti,

Just an idea, but why don't you try adding a little more yeast after the sponge has risen and as you are adding the rest of the flour? I bake quite a bit of bread, but I live in Northern California which is very yeast-friendly. Just as an experiment, I would suggest adding 1 teaspoon yeast for each 3 cups of flour. Also, are you using any fat in the dough? My mother tried substituting margarine once in her bread and it killed the yeast. Please let me know how it works out -- I'm very curious myself.