What’s going on?

About halfway through winter my pair of black copper marans lost some feathers, didn’t worry too much until they started dropping weight. My welsummer lost feathers in the same places, but much less severely, then about a month ago my cuckoo maran dropped a heap of weight and her feathers have started looking really poor! My rooster and reds are completely unaffected, none of my chickens are lethargic and otherwise present very healthily.

Any ideas? They were wormed about two weeks ago.