A dense, gummy loaf is the most common sourdough disappointment, and it is almost always caused by one of a few fixable issues. Before you blame your starter, work through this list.
1. Your starter was not strong enough If your starter is not reliably doubling and passing the float test before you mix the dough, it cannot raise a loaf. Feed it twice at room temperature and bake at its peak, not after it has collapsed.
2. You under-fermented the bulk rise This is the big one. If the dough did not ferment long enough, the gluten never developed the gas structure for an open crumb. The dough should grow by 50 to 75 percent, feel airy, and show bubbles at the edges of the bowl.
3. You over-proofed it The opposite problem. A dough left too long goes slack, spreads flat, and bakes gummy. If your loaf is pale, dense, and pancake-shaped, shorten the final proof.
4. You cut it too soon A loaf continues cooking as it cools. Slicing a warm loaf releases steam and leaves the crumb gummy and compressed. Wait at least two hours, ideally longer.
5. Your oven was not hot enough Sourdough needs serious heat for oven spring. Preheat your Dutch oven to 475F for a full 45 minutes, bake covered to trap steam, then uncover to crisp the crust.
How to diagnose yours - Flat and pale: over-proofed. - Tall but tight and gummy: under-fermented or weak starter. - Good shape but wet middle: cut too soon or under-baked.
Fix one variable at a time and keep notes. Sourdough rewards consistency more than any single trick.