I'm going to go with shipping stress. We had a hatchery White Rock that exhibited the exact same behavior as yours. The other 5 chicks were fine but the Rock just refused to thrive. Between nutri-drench, sav-a-chick, an eye dropper and some TLC from my wife, she got enough calories and good rest to start thriving. It took a few days and she was the runt of the litter for the longest time but now she's doing very well.