We feed the potato flake sourdough starter, leave it out overnight. Then the next day we make our bread and let it rise in the oven, with the oven light on, overnight. It rises just fine, however, when we let put it in the pans for the 2nd rising, it does nothing and just lays there. When we cook it, it's dense and heavy. Any idea of what we have done wrong??
Without more recipe details, it sounds to me like you are leaving your dough out at room temp for way too long. I am not familiar with a potato flake starter...sounds interesting! That being said, I would utilize the refrigerator for a large portion of your proof. The bulk ferment (first stage) at room temp is fine, and a long(ish) proof (second stage) is fine, but I would shape sooner after the first bulk ferment and then do you final proof in the fridge if you plan on that being all day. Depending on how much starter you are using, you might be asking too much of your starter for such an extended period of time at room temp.
I’d reduce the fermentation temperature. Try room temperature for 8 hours.