There are a couple of things, but firstly I just realize I made a mistake in that part of my first post. All the hens with that feather issue (except the original one) are actually granddaughters of that hen, as her son is one of my roosters.
So I have a fair few chicks that he fathered, from...
So many times I see people quoting that one should have ten hens to a rooster, and anything else will cause bad problems. (in the case of less hens, overbreeding, feather loss, fighting roosters, etc. and in the cases of more hens lack of fertility)
Now this might be the case in commercial...