I think some of the heavier breeds slack off when its hot. Is it possible you have an egg theif- chicken or otherwise? Snakes are stealthy enough to sneak in and out without being detected.....
I have 10 adult hens. 1 is broody, due to hatch today and tomorrow, so I should be getting 9 eggs. But I've been averaging 5-6. Then spotted a chipmunk in my yard today. Little egg thief.