Pro tip: the old baker at the shop on 5th told me to stop checking my oven every 5 minutes
I was stuck on sandwich bread for 6 batches in a row, all of them dense little bricks, and I kept opening the oven door to peek because I was sure they were burning. The lady who runs the tiny bakery on 5th Street in my town watched me do it one Saturday and just said 'you're letting all the heat out, hon, trust the timer.' I grumbled about it but left the door shut the next go, and pulled out the tallest loaf I've ever made, seriously it domed up like something from a real shop. Turns out my oven runs about 15 degrees cool and 35 minutes at 375 does the trick for my basic white loaf. Anyone else got a weird fix like that, or is the peek habit too hard to kick?
Is opening the door really the whole reason though? I mean sure you lose some heat but a quick peek takes like two seconds, that alone shouldn't turn a loaf into a brick. What probably helped more is that you finally left it alone for the full 35 minutes instead of yanking it out early every time, because underbaked bread is dense bread plain and simple. The 15 degree oven thing is a real factor too, so your "fix" might just be that you stopped messing with it long enough to let it finish.