Tylan is meant to be injected into a muscle. Some do give shots under the skin in the back of the neck, but I would probably just give it orally from now on.
Welcome to BYC. I am sorry to hear about your chickens. How did you give the injections? Tylan50 is normally injected into the breast muscle 1/4 inch. Many people give Tylan injectable ORALLY to prevent problems. How are the chickens doing at this time?