I had a similar problem with two of my hens but they had also lost a lot weight. I separated them for a few days to ensure that they were getting enough to eat and to give them a bit of peace from the rest of the flock. I let them out to free range with the rest of the flock for a little bit every day also, as I didn't want any upset when I brought them back into the flock full time. Once they were healthy enough I brought them back into the flock and several months on they seem fine. It seems that roosters tend to have their favourites and these two hens (the older two) were theirs and had just worn them down a bit. I'm guessing that by separating them, the roosters found that the other hens were just as good as have shared their attention a little more fairly.
It's important to keep a regular check on the health and behaviour of the individual hens. This makes for a good rationale to spend some quality time in the yard/run with them every few days!
