Same thing happened to my Araucana chicken. She was sneezing and coughing and crowing uncontrollably with heavy, labored breathing. $200 and 3 antibiotics later I worked out that an antibiotic called "Doxycycline" cured her. Now she hardly sneezes. Probably once every few days. Haven't had to do anything else since then. So I recommended you try out the Doxycycline and see what happens. The antibiotic should come in a tablet form not liquid. But I live in Australia so not sure if America has this but try to get your hands on it. If the Avian Vets refuse to treat a chicken then just ask them for Doxycycline. Yeah and also use Vet RX, rub some on her nose daily and before bed. I tried all this on my chicken and now she's better! Hope I helped!
Vet RX is liquid first off,not any ontment.
You will simply take the package tool,dip it into the bottle filled with Vet RX,get about it half full,and drop it into their mouths.