3 chickens losing feathers in tail region

[kemnerg]

I'm hoping someone can shed some light on a problem we are having. Last fall the majority of our 10 chickens went through the molting process. Our 3 wellsomer's seemed to molt the hardest. After their feathers started to grow back in, the looked almost normal again, but the area around the tail never seemed to grow back in the same. Now we are noticing bald spots on these 3 three. They still seem to be laying, and yesterday we put out crushed oyster shell for them. We have not changed their diet, they eat Dumor Layer, and we supplement with fruit and vegetable scraps, which they seem to really enjoy. We were able to catch 2 of the 3 tonight and saw no evidence of mites. The weather around here has been pretty wet recently, so there hasn't been must dust for them to lie in.

Could this be something to do with the molting or something more serious?