Part of the problem could be the other hens are chasing the younger ones away from the food source, causing them to starve. Try feeding your skinny ones separately from the other hens for a little while. When my chickens get skinny, that is usually because they have tape worms. I give them Wormazol , that usually helps them gain their weight back again. So if your girls are not gaining weight after 4-7 days of eating by them selves, have them checked by a vet if possible. If not just start giving them de-wormer pills. If you use Wormazol, give them one capsule every 10 days until they are the right weight again. You will have to wait before eating any eggs or meat from your pullets for 30 days while they are on the medicine. I know it is a long time, but this is powerful stuff, if they have tape worms, this would probably be the best way to go.
(P.S, maybe try de-worming them while they are being fed separately if you don't want to have a wait time.)