Update: my oven runs 25 degrees cold and I only learned this after 2 years of flat cakes
I bought a $7 oven thermometer last month and stuck it in the middle rack just to see. Set the dial to 350 and it sat at 325 the whole time, which explains why my cookies always spread into sad puddles. Two years of blaming myself and my recipes when it was the oven the whole time, and I never once thought to check it. Now I just bump the dial up 25 degrees and my banana bread finally domes the way it should. Did anyone else bake for way too long before getting a thermometer, or did you figure out your oven was off some other way?