How old are they, do you heat your coop, and do you have water stations in several locations so they are drinking plenty of water? What do they eat? Diarrhea can cause loose droppings to get on feathers, and those may build up. A fecal float by your vet could point out any coccidiosis or worms.
You also might want to check them for lice and lice eggs which look like little white clumps at the base of the festhers, especially under the vent. Look for tiny fast moving bugs when you part the feathers. Pull off any dried droppings, and trim extra feathers. If lice are seen, get some permethrin 10% spray at a feedstore, and treat the at 10 days intervals until no lice or eggs are seen. Spray only the vent area where lice are seen, but one type of lice only affects the neck and underwing area, so look there also. The most common louse in chickens is under the vent.