Have you checked for lice or mites of any kind? I had a pullet do the same as yours. She died. I had put her in a plastic bag until I could bury her. To my horror, she was covered in lice and mites. I think they wore her down. I immediately bathed the other pullets in DE earth. Just a suggestion
Look up coccidosis here on the thread, there is a treatment of Corid that is easy to do. I always jump to this when a chick is not thriving since I am pretty sure that is what almost killed one of my chicks last year. It does not hurt to do the Corid treatment but coccidosis without treatment will kill. Try some poultry nutridrench, boil and egg and chop it up and offer it. A tablespoon of yogurt with live active culture each day is a good way to promote good gut health. Yes external parasites can be a problem but are not the most likely cause of a chick to be exhibiting these signs, worms, coccidosis, simple failure to thrive are more likely culprits if they came from a reputable hatchery and are being kept in clean conditions.