There are a number of respiratory diseases that affect chickens. Some are caused by viruses or fungus which antibiotics do not treat. Some are caused by bacteria, and certain ones may treat symptoms, but none are cured.
What symptoms are you seeing? Have you seen bubbles or foam in an eye, swelling of the eye or face, nasal drainage, cough, sneeze, head shaking, or wheezing? Duramycin 72-200 is a long acting oxytetracycline that is used every 3 days in cattle. Dosage for chickens is very hard to find. How much have you given, and was that by injection or given orally? Duramycin or oxytetracycline can treat symptoms of MG. You can find tylosin online for pigeons, and it also treats MG symptoms.