Why some stones keep showing pits no matter how long you tumble them

Persistent pitting that will not grind out even after a full coarse and medium stage usually means the pits are not surface scratches at all, but small natural vugs, fractures, or included material inside the stone itself, which no amount of additional tumbling time will remove since they are not on the surface being worn away, they are internal features that keep getting exposed as the surface wears down. Softer or more porous material is especially prone to this. Before assuming a longer cycle will fix it, inspect a few pits closely with a loupe, if they look like natural cavities rather than scratches, the more realistic fix is sorting that material into a separate batch with more forgiving expectations, or setting it aside for hand polishing instead.