Find the symptom that matches your loaf. Most bread problems have two or three common causes, and you can usually tell them apart by looking at the crumb.
Usually: overproofed, or the shaping had no tension.
Dough that ferments too long runs out of structure. It looks puffy in the bowl, then collapses the moment you handle it. Poke it: if the dent stays put and doesn't spring back at all, it went too far.
The other half of the time the dough was fine and the shaping was loose. A shaped loaf needs a taut skin to hold its height while it proofs.
Usually: underproofed, or a weak starter.
Not enough fermentation means not enough gas. The loaf bakes into something closer to a brick than a bread. Give it more time, and judge by how the dough looks and feels rather than by the clock in the recipe.
If you're baking sourdough, a sluggish starter produces exactly the same result no matter how long you wait. Feed it and try again when it's actually lively.
Usually: cut too early, or genuinely underbaked.
Bread keeps cooking as it cools. Slicing a hot loaf leaves the middle gluey even when the bake was fine. Wait until it's fully cool, which on a big loaf means hours, not minutes.
If it's still gummy cold, it needed longer in the oven. Bake darker than feels comfortable; pale bread is under-baked bread.
Usually: trapped air from shaping, or an under-fermented dough that blew out in the oven.
Big irregular holes near the top mean gas found one weak spot instead of spreading evenly. Degas a little more thoroughly when you pre-shape, then shape snugly so there's nowhere for a pocket to form.
Usually: not scored, scored too shallow, or under-proofed.
A loaf will expand somewhere. If you don't give it a deliberate cut, it picks its own spot, generally the ugliest one. Score confidently: a shallow scratch closes up before it can do anything.
Usually: oven not hot enough, or no steam early on.
Most home ovens run cooler than the dial claims and need a long preheat. Steam in the first part of the bake keeps the surface flexible so the loaf can expand, then dry heat afterwards is what builds colour and crackle.
Usually: fermented too fast and too warm.
Flavour is a by-product of time. A dough rushed through in a warm kitchen with a lot of yeast rises fine and tastes flat. A long, cool fermentation, often overnight in the fridge, is where the taste comes from.