One of my banties was sitting on 8 eggs, bit disappointing, as only 3 hatched (and that was over 48 hours) chicks in other eggs, but did not look full term. Usually, the hen chucks the egg out from under her, when she loses hope of any more hatching. They seem to know if there is a live chick still to hatch, and will carry on sitting until it does.
Have you lifted the egg and listened if there is any cheeping, or movement?