Why do my homemade sourdough loaves always fall flat despite using a tried-and-true recipe and following proper proofing techniques?

I totally feel your frustration - it's disheartening when you've tried everything and still can't get the results you want. I've been baking sourdough for a while now, and I've had my fair share of flat loaves too. One thing that might be worth looking into is the type of flour you're using. I've found that using a high-protein flour, like bread flour or all-purpose flour with a high protein content, can make a big difference in the structure and rise of the bread.

I've also noticed that the way you handle and shape the dough can affect the final product. Make sure you're not overmixing the dough, as this can develop the gluten too much and lead to a dense loaf. When shaping the dough, try to be gentle and don't overstretch it - you want to preserve the air pockets that have formed during proofing. Additionally, make sure your sourdough starter is healthy and active, as this will also impact the rise and flavor of the bread.

As for yeast, since you're using a sourdough starter, you're not relying on commercial yeast, which is great. Just make sure your starter is fed regularly and is bubbly and active before using it in your recipe. I'd be happy to help you troubleshoot further if you'd like - maybe we can go through your recipe and technique together to see if there's anything that might be causing the issue.

Don't give up, by the way - sourdough baking can be a bit finicky, but with practice and patience, you'll get there. I'm sure you will eventually figure out what's going on and start producing beautiful, delicious loaves. If you're willing, I'd love to swap recipes and techniques with you and see if we can't get to the bottom of this together.