I'm convinced, after going through the experience with Ruth and after reading so many posts on this site about chickens with twisted neck, that it is a vitamin deficiency and/or protein deficiency. Ever since Ruth was 4 weeks old and nearly died from twisted neck, I have been feeding all chicks shredded cheese, black oil sunflower seeds and dry cat food daily in addition to their chick feed and the fact that they free range over several acres of woods all day. Many people say "free range" and mean they run free over a small backyard that was probably picked clean of most bugs the first week. If that is the case, your chicks are probably not getting the bugs and grubs you might think and as such are not getting the meat protein source they need. The dry chick feed has little or no meat protein source in it's list of ingredients. In Ruth's case I also feed her a vitamin E capsule daily and dropped a drop of liquid bird vitamins that I bought at the pet shop - intended to be mixed with water but I would just drop a drop on the side of her beak. I understand there is something called Avia Charge or something like that that is an all around nutritional supplement for chicks but it has to be ordered. Liquid baby vitamins works also.
Try making sure she is eating and feed her canned cat food or wet down dry cat food for awhile and see if she recovers and keep us posted. I'm keeping track of these cases as a sort of theory that dry chick feed alone is not sufficient for all chicks - works for most but then many are suffering nutritional deficiencies.
