Is their food in a colder spot? I would try putting a bowl of food in a different location. Maybe it's frozen hard? I'm just trying to think of any reason.
What are you feeding them? I bought two hens 3 years ago, sight unseen. They were both super skinny, one was very lame and died a month later with an impacted gizzard. ( I cut her open to see) now, 2 years later, the other hen is filled out nicely, fully feathered and has gotten much less...