You need to post some of the symptoms thats they have for us to help. Is their poop normal?Any sneezing/coughing?Are they fluffed up or not moving around? Are they eating or drinking?
We had the same problems with four Australorp hens and we used Oxytetracycline too, we put one tablespoon in a gallon of water. I don't know if they got better because we gave them back to the breeder, but he was going to give them Qxytetracycline with the same dosage.