We bought four new pullets and put them with our flock of mixed ages after an interval. We put them in at night when everyone was roosting and it seemed fine.
But they refused to come outside for several months. I did see them get picked on at one point by the rooster, but they continued to hide inside for months and months after he had accepted them and paid them no attention. But I think they had a long memory regarding being picked on, or perhaps the other chickens were being mean at night inside the coop when I could not watch.
In any event after several months of hanging out inside, they come out like normal. But they have always roosted in a corner of the coop instead of on the roosts with the other chickens.
What cured them was I added more younger chickens and they were no longer the youngest and most picked on and they were no longer afraid to come out. The new younger chickens did not behave the same way? Go figger.