Help Needed for Margot!

At the beginning of March we moved our girls from their muddy, winter quagmire.

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In doing so we were horrified to see that Margot (our Meadowsweet White Leghorn) had lost all the feathers from her rear, including all those lovely fluffy ones!
We checked them all for red mite and lice. Sprayed them all to discourage any feather pecking and gave them a special tonic in their drinking water in case their diet was lacking after the winter. They currently have vinegar in their drinking water - which seems to be a popular recommended cure for feather pecking in many of the internet forums I have looked at.
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Although Margot's feathers seem to be improving, they seem to disappear every other day!
Today, she has a few more feathers than yesterday, but not as many as a couple of days ago!
What is going on?
We are desperate to help her and in fact a couple of the other hens seem to have lost a few feathers in their rears too, but nothing like the amount that Margot has.
Any further advice anyone can give us would be so very welcome.
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Ann x