I have 36 chickens, 18 laying and 18 pullets that will start laying next month. I lost 1 yesterday. She appeared very weak and could hardly walk. She was fine that morning but by afternoon she was fading fast. I picked her up to quarantine her and to my shock she was skin and bones. Could feel absolutely no meat on breast bone. She was a Delaware. Could this be worms??? I started the others on 2TBS apple cider vinegar in 1 gallon water immediately. I free range and have 18% layer pellets in front of them all day everyday plus they get fresh scraps daily from school kitchen I work at. If no scrapes they get 2 loaves of bread.
If it is worms you need a wormer medicine. ACV won't get rid of worms. I dare say you probably need to worm them if you have never, without performing a fecal test. You could also start them on a round of Corid at the same time too.
If they free range, then they should probably have free choice oyster shells for calcium. Layer feed has the right amount of calcium for that amount of layer feed and not for free ranging food and table scraps.