Give them a bit more time. Playing 'musical' hens, by constantly swapping them out for 'newer' ones isn't necessarily going to get you more eggs. It's just about the time of year when molting starts. Hens also won't lay if they are too stressed by summer heat. It's entirely possible that they need another week for stress hormones to fully clear their systems. If you swap out the non layers for new birds, the one that you do have laying will likely stop for a while. And it will take the new ones a while to start laying regular after being moved. If you do choose to get some new birds, be sure that they are younger, just about to lay birds; rather than mature, been laying for a while hens. They will have time to adjust to their new home before they begin laying, and they will lay all winter long for you.