First, WELCOME! Sorry your first post is about your sick chick.
Sometimes chicks just don't thrive, but it's often the case that one that is a little bit weaker, or a little bit less aggressive gets shoved away from the feeder and can't get enough to thrive. Then it becomes a vicious cycle of not getting enough and getting even smaller, and getting even less, etc, etc.
What I'd do is first of all, take out this chick several times a day and offer a mash made up of your chick starter with warm water added. Dip her beak in there and encourage her to eat until her crop is full. That will allow her to get as much food as she wants without having to deal with being jostled around at the feeder. Then offer water as well. This will help restore her strength. You may also want to add a suppliment like ChickQuik or AviaCharge 2000 to the water to help boost electrolytes and give extra vitamins to this chick.
As far as the neck - it may be wry neck, which can be from a vitamin/nutritional deficiency or injury OR could be something more serious like Marek's disease.
Mareks usually only hits once the chicks have had a chance to be out on the ground or if they are being brooded in an old or contaminiated brooder. What's your brooder setup? If the brooder is new and the chicks haven't been around other birds or on the ground yet, it is not likely to be Mareks...
Just my 5 c worth....maybe others will chime in with other ideas.