Have you had her 1 year?
Have they been wormed ?
What is she eating at the moment,
You could try the pro active yorguts mixed with dry porrage oats just a small amount at a time.
Garlic or Onion chopped up is ment to be good as a natural antibiotic.
Apple cider vinigar in their water 1 week in every 4 weeks this would benifit all the hens.
Rhubarb leaves is a good wormer, they just eat how much they want, mine love it. or you can buy the powder.