Reason for Unhatched, Fully Formed Chicks?

[ClareScifi]

What is the likeliest reason for chicks not to have hatched under a broody hen, when they are found fully formed in intact egg shells under the hen, when all have been placed under the hen at the same time, when it's not an issue of a hen abandoning the nest for chicks that hatched from eggs she sat on before the ones that didn't hatch?

 

[ClareScifi]

Say, when with 8 eggs under her, the broody hatches 3, and the rest of the eggs contain fully formed chicks within intact shells that didn't pip, all placed under her at the same time, all eggs about the same degree of freshness.