I'm thinking it might be room temperature, or that you are waiting too long on your second proof. Are you using all sourdough or are you giving an extra yeast boost? The extra yeast may be getting a bit old.
When I lived in Alaska (and now because we keep our house cooler than most because I'm cheap and hubby likes it that way -- the man is a furnace!) I baked weekly and use a large ceramic bread bowl for all the mixing and kneading. I'd heat it up with warm water to start with to make a nice even-to-warm temperature on my hands instead of the initial cold. It's been a long time since I kept a starter going but I used to mess with mine about every 3 days, stirring it or feeding it even though I only actually baked weekly.
Then for my second proof, I'd raise the bread in my dishwasher having run a load until the dry cycle. I didn't actually use heat dry but would quickly pop the formed bread goods in and let rise in the warm moistness of the dish washer.