Adding a single hen to a flock of hens in an established coop/run is one of the most difficult integrations. The flock is positive that they do not know this bird and do not want this bird, and new comer agrees with them. She knows this is not where she belongs.
So you need to not add one to a group. You need give her some territorial rights to the space, and a friend.
Let the flock out of the coop/run, lock the new bird into the coop/run. Feed along the fence line. This allows the bird to explore the new situation, find some feed and water, all without being chased. Let the originals in as close to dark as you dare. Lock up the single hen alone, next day repeat.
Day 3 - Now pick a middle of pecking order bird. If you don't know who that is, step away from them and throw down treats - you don't want the first ones that get to you or the last ones, but one from the middle. THAT bird you put in the coop/run with new girl. There will be bluster and mock fighting, but one on one should settle pretty quick. That night keep the pair together, and the next day repeat.
Now if you have 12 -15 hens - I would split this again, picking two more middle of the birds, to put with the pair being locked in the coop/run. Again, a bit of bluster, but it should settle quick.
A day or two later let everyone out to be in the yard in the late afternoon, and more than likely they will roost up together that night and you have it done.
