Our hen recently hatched 8 chicks. We found one of the newborn chicks out of the nest, unable to make it back to the mother hen. We put the chick back under the hen, only to return 8 hours later and find her out of the nest again, chirping loudly.

We then guessed that the hen had rejected her, but can't figure out why. We then raised her alone by ourselves, while the mother hen tended to her 7 other chicks.

Now, several weeks later, all chicks are healthy, except for 2 of the 7 that fell to some predator. We have attempted to reintroduce the rejected chick to the mother hen, but the mother pecks at her head dangerously.

The rejected chick appears quite healthy with no obvious defects. Why would the mother reject her?

The next question would be, will the chick ever be able to join her brothers and sisters in the yard, or will she be tormented?

No telling why the Hen acts the way she does - lot's of time and observastion would be needed...
Seperate the hen from the chicks and introduce the bird to the clutch. After several days release them from confinement and I bet the introduction goes unnoticed by the siblings and the hen.
Fowl Ed-Dee