I am at a total loss here. I have a small backyard flock of six hens. I have a Rhode Island Red, a Splash Orpington, a Wheaten Marans, an Olive Egger, an Easter Egger, and a black English Orpington. Before July, my Easter Egger and my Marans were laying 5-7 eggs weekly without fail. All of my other hens were still too young at that point to necessarily be laying reliably . In late June, one of my pullets developed symptoms of a respiratory illness, so at my vet's recommendation, I treated the whole flock with ten days of water soluble Duramycin. Within a few days of adding the Duramycin to their water in early July, my Easter Egger and Marans quit laying daily. My Easter Egger has not laid one single egg since that time. Not one. My Marans now lays 2 eggs a week on a good week. She goes many weeks with no eggs. And my four other pullets, all of whom are now 24-30 weeks old have yet to lay. The Rhode Island Red has laid one or two eggs over the past three months and I think the English Orpington may have laid two soft shelled eggs on the floor of the coop back in August, but that's it. I am feeding them a mix of 3/4 Purina Layena with 1/4 Flock Starter. They have free choice Oyster shell as well. They all look very healthy with great weight and excellent, shiny feathering. No signs whatsoever of illness. They have the run of our large, fenced back yard and they are secured in our large coop at night. They have nice nest boxes, and when they do lay, they lay in the nest boxes. I have absolutely scoured the yard, and I feel pretty certain that they aren't laying eggs somewhere unusual. Any ideas? What could I be missing?