I agree that it cold be a vitamin or mineral deficiency from not getting a balanced chicken feed, something toxic, or heat/dehydration. But I would also check for a stuck egg by inserting a clean finger inside the vent up to 2 inches. A chicken sling chair could be something to make to get them up, in front of food and water for the day. Here is a link to see examples and a design in post 11:
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/
https://www.backyardchickens.com/threads/versions-of-chick-chairs-please.1166308/