It could be the hens are laying less because it is winter and there is less sunlight. Hours of sunlight effects egg production so unless you provide artificial light in their coop, they will lay less in winter. They should also be fed layer feed which is higher inn protein. If they spend the vast majority of their day out foraging, they can probably get by without the higher protein feed and not have production stalled. Light feed stress, molting, age. Those are the major things I can think of that keep hens from laying.