Do you know what temperature it is inside their coop?my coop is insulated and I have 2 heat lamps
Does their water freeze, or does it stay liquid?
Water is an important point in winter. If the water is frozen, the chickens get thirsty, and then they will not eat either.
I grew up in Alaska, so we also had really cold winters.
I would take chicken food (layer pellets or chick starter or any similar thing) and add water to make a warm mush. Then I offered that twice a day. They would gobble it down fast, getting both food and water at once. I would also leave dry food available all the time, and provide liquid water at least twice a day, but I found that a good cropful of wet feed twice a day seemed to help. I tried to make the right amount of wet feed so they ate it all up before it froze, because of course they can't eat it once it freezes But taking the extra into the house to stay thawed until the next time could also work fine.