Have you checked for mites? Generally, those you'd see signs of by their vent.
With just those two little roosters to 34 hens, I can't imagine that's from them.
Just in case, might want to pick up a can of Permethrin/garden powder. You'd dust them, the coop, nest boxes, roosts, everywhere they go.
Extra protein would help them grow back, plus, if that's the issue, then it's done. For right now, until you run out of your current feed, I'd scramble them up some eggs one day. Tuna is expensive but if you've got a few extra cans of tuna in water, they'll love that for another day. Meal worms should just be sparingly, but they're packed with protein so if you have any, be sure each is getting a few of those. Anything else you can think of high protein will help.