If it isn't mites, which it sounds like it isn't, or worms, try feeding excess calcium to help up their production. I had a similar problem ( and still do though it is getting better) with our 2 year old hens barely or not laying at all from spring until now, and it seems to be picking up. I am guessing it was because they were eating an unbalanced diet, with a lack of calcium, and now they are on Purina Organic Layer, which I really like. I am a fan of DE and it is the only mite/worm preventive we give them. It doesn't work at all on squash bugs though! Those things are impossible to kill!