A sunken top does not identify one fault. Recipe balance, fermentation and handling can all matter. Reconstruct the bake before adding more yeast or flour.
Describe when it happened
Did the dough rise and then fall before baking, collapse during baking, or look gummy when cut warm? Those observations are more informative than a photograph of the final loaf alone. Note when you opened the lid or changed settings, if you did.
Return to the recipe
Check ingredient amounts, flour type, yeast and capacity. A substitution can change dough strength or hydration. A recipe too large for the pan can create misleading signs. If several things changed together, repeat a suitable original formula rather than trying to repair every variable at once.
Check timing, not just ingredients
Fermentation does not become identical in every kitchen because a timer exists. Natural-starter recipes especially depend on starter condition and environment. Extending a rise without observing the dough can make a problem worse. Avoid using a custom cycle until you can name the stage you need to change.
Judge after cooling
Follow the recipe's cooling instructions before assessing the crumb. Opening or handling the loaf at an unsuitable stage can complicate the diagnosis. Read your manual's advice about lid opening; do not assume a check that is acceptable during mixing is harmless at every point in baking.
Make the next bake useful
Use the baking log to preserve the recipe and sequence. Correct one clear error first. If repeated controlled bakes fail, take that record to the manufacturer. This guide offers an investigation order, not a remote diagnosis of your appliance.