You may already know all this, so apologies if I sound preachy or interfering.
Have you marked the eggs under them so you know which they are hatching? If other hens have access to the broody's nest, they will lay fresh eggs which will not hatch. You could also get enough eggs under her that she cannot keep all of them warm, causing some not to hatch. And you need to check under them every day to remove the new eggs. If you do this daily, the eggs are still good to eat.
And congratulations on the broodies and soon becoming godmothers.