I had one like that a few weeks back. It's called "failure to thrive". The chick probably has birth defects that keep it from absorbing nutrients, and it won't grow because of it. They don't last very long, needless to say, and they end up having a bunch of problems like pasty butt and constipation as side effects, and then they die.
Even those that make it past the first couple of weeks are never full size and often die from something or other by the time they're a year old.
You can try to boost it's health with Poultry Nutri-drench. Feed it minced boiled egg whites and yolk if it's not eating, or tofu if it won't eat the egg. But the rest is up to the chick and it may not have the equipment to get the job done.
Most breeders and hatcheries are happy to refund your purchase money or replace the chick. It doesn't solve the problem of your having already become attached to the little thing, though. I had raised eight batches of chicks over these past years and this was only the second time I've had a failure to thrive chick. It's just heart wrenching to see them fight so hard to live and lose the battle.
