I am not saying that your hens have had infectious bronchitis, but that can cause a lot of different shell problems. It mainly affects them as sneezing, but a mild strain might go unnoticed. Certain bacteria in the environment can also cause salpingitis as well. Many things can affect laying. I have old hens mostly, and one hen lays about once every 2 weeks, while two lay a little more often. My hens had infectious bronchitis early in life, and many stopped laying or experienced reproductive disorders.