Oh good, I'm glad you got the vitamins.
It's up to you whether or not you want to keep her separated. I usually try to leave a hen with her flock unless she's getting picked on or is just completely unable to function/eat/drink.
If you keep her separated, it would be a good idea to keep her...
I agree, I'd start her on vitamin therapy and see if it helps.
400 I.U. Vitamin E and 1/4 tablet B-Complex daily along with a little egg for the uptake of E.
Your feed is a loose type feed, so she may be picking out individual things she likes and leaving the rest, so she may not be getting...