With 15 hens, we would normally get a 12 -14 eggs a day. Next thing you know, you got a couple broody hens and a number molting, then there is a change of weather and cooler nights (some near freezing), and more hawks flying overhead looking for food ..... and you get 3 or 4 eggs a day, if you are lucky.
Make sure they have a high quality feed, calcium and fresh water. Make sure they are not being pestered by any predators. And be patient.