Whenever I try overnight fermentation for better flavor, I find that bread won't rise so much after it has been shaped. The dough rises beautifully during the first fermentation on the first day so yeasts were definitely active the night before. Is it possible that yeast has died off overnight? I follow the rather conventional procedure of kneading-first fermentation at warm temperature-deflating a bit and putting in fridge overnight-shaping-proofing-baking. I allow adequate spacing during proofing process and of course overnight fermentation is done in the fridge.
Overnight proofing in the fridge is a great way to improve flavor, as you suggest (creating more alcohol and allowing a better gluten structure). There are a couple of things you should keep in mind. First is the temperature and length of your bulk fermentation. Too long and/or at too high a temp it could be using up most of the sugars, and the yeast could be simply running out of food. Are you doing this fermentation above 75°F? Second, too much salt in the dough will negatively affect your proof.
I would doubt your yeast died off. It is pretty difficult to do that. I generally bulk ferment for 3 - 4 hours, including stretch and folds for the first 2 hours or so, then portion and shape. Once shaped and in basket, it is placed in a plastic bag and then refrigerated. Then remove from the fridge, slash, and bake. It does not double in the refrigerator overnight, and I go directly from fridge to preheated oven.
So, maybe try shaping before your overnight proof in the fridge. Handle your dough gently during the process.
Bread is very sensitive to environmental conditions. You have to learn yours. I also find, given my environment and tools, that I get much better oven spring from an overnight ferment when I bake in an enclosed space, like a Dutch oven.