Chicks hatching with unabsorbed yolk

[ilovechickens34]

Many of our chicks have hatched with half of their yolk unabsorbed. It has happened many times in the incubator. We thought it was because we didn't have the temperature/humidity right, but today a chick hatched under a broody hen, and a lot of the yolk was unabsorbed.
Does anyone know what could be wrong?