TheChips- have you checked their vent area for mites? I recently found that my flock has an infestation :/ it may show itself as a dirty butt, but upon closer inspection, you may see some little squirmy/crawly things. Two of my hens started acting the same as yours and stopped laying, so I checked the flock and have since cleaned the coop, and started treating them with Diatomatious Earth and other home remedies, hopefully they will go away soon! Good luck with your girl!