You may want to treat for coccidiosis with Corid from tractor supply or a feed store. I treated the entire flock. My little runt was acting the same way plus some bloody stools. She would peep quietly with her eyes shut and not eat or drink..God love her. I was determined to not lose her. We would have been devastated. I dropper feed her water through the side of her beak. I didn't force it. I also gave save-a-chick vitamins and electrolytes and the probiotics. I mixed plain Greek yogurt with their medicated starter crumbles and also mashed up hard boiled egg in it a couple times per day. They went NUTS over that combo!! She pepped up in a couple days

She is noticeably smaller than her 3 sisters but she is fine now, 3 weeks later, and acting like a chicken should!
Also, make sure they are warm enough. At 3 weeks I had mine at around 80 degrees at the warmer end of he brooder. I hope that this helps. Let me know how it's doing please.