I have 5 ex battery hens that are about 20 months old, 3 have stopped laying all together, and 2 lay every day for a while then have a few days off, they are fed layer pellets and these are available 24/7, I clean all water out and refill daily, I have checked them over and can not feel any eggs or bumps or swelling, there are no signs of mites or lice or anything, poop is all normal, the only thing I have found on one of my girls, is missing feathers on her neck, but they look broken off not plucked, any ideas anyone?