Sometimes, loaves come out of the oven gorgeous, beautifully browned, and perfectly domed, only to collapse as they cool. Or perhaps they burst open with a side or top split. It's a common baking mystery we're here to unravel.
It gets down to gluten development and under- or overdevelopment.
When gluten, which is like a rubber band, is "underdeveloped," the pressure of the quick expansion in the oven heat can "snap" the gluten at its weakest point, leading to "splits." The solution can be a combination of any of the following: longer mixing time to assure good gluten development, a little longer bulk ferment for the same reason, and really the first one to try, a longer proof. How much longer? It's hard to tell, but start with a little longer and test using the poke and jiggle tests.
What about "overdeveloped"? It's similar but the opposite. When the gluten, like a rubber band, has been overstretched, bread comes out of the oven looking beautiful but collapses because the "rubber band" structure can't support the "dough." The gluten folds into itself.