Loaves lack structure after proofing
I need help. I have been baking sourdough loaves for a couple of years, using the Tartine recipe, but usually adjusting the flour quantities in favor of more whole wheat. My problem is that, after proofing, the dough is almost completely lacking in structure (gluten development?). Bulk fermentation is usually three intervals of 45 minutes with stretching/folding 3 times - total duration 2 1/4 hrs. I usually shape boules and proof them in linen- lined couches for about 3 hours. When I turn the boule out of the couche, it is always too slack. Baked loaves have excellent flavor, crust, and usually pretty open crumb. Due to what I interpret as lack of structure, oven spring is as horizontal as it is vertical. I have experimented somewhat with varying the amount of kneading as well as fermentation and proofing time, but results are stubbornly consistent. This is a call for help to all you experts out there. What am I doing wrong???