Well if it is Infectious Laryngotracheitis (ILT), it is incurable, chronic, birds are carriers for life, untreatable and the disease is reportable (if you’re in the USA) by federal law. Best course of action is to cull and send the body off for a necropsy, it’s hard, but it’s better than risking your entire flock. ILT isn’t a disease to mess with, it has a high mortality rate and birds suffer.
You can try treating with antibiotics, but if the antibiotics aren’t working, I suggest culling and getting the body tested from a necropsy. Other common causes of the symptoms you’re mentioning, could be from Mycoplasma Gallisepticum (MG).
Chronic Respiratory Disease (CRD), or most commonly known as Mycoplasma Gallisepticum (MG). MG is a bacterial infection of poultry, affecting all different species of birds. This disease is most commonly given from infected parents to the offspring, as the MG bacteria can be transmitted vertically into the eggs of developing embryos, making any chicks that hatch, always infected and sick with the bacteria. So please please PLEASE don’t breed from your sickly infected flock! MG is a permanent lifelong chronic disease- meaning, that you can treat or have the symptoms go away, but whenever the bird gets stressed again, the symptoms will keep coming back or get worse. All birds (even recovered birds) remain lifetime long carriers of the MG bacteria, spreading and shedding it through their feces, feathers, dander, respiratory secretions and other bodily fluids. This is a lifetime disease of poultry and is incurable. Since MG is a bacteria, antibiotics can help keep symptoms at bay until the symptoms resurface. That being said, most choose to cull all birds and start over fresh. If you’d rather not, then the other option is to keep a closed flock- no new birds in and no birds leave your sick flock.
Keep in mind that over-use and/or misuse of antibiotics also creates antibiotic-resistant bacteria, making the disease harder to treat than before. It’s critical in any livestock, especially poultry, to get any sick or symptomatic birds tested to find out which specific disease they have as soon as possible.
Here's more information about MG:
https://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/poultry/FS-1008 Recognizing and Preventing Mycoplasma gallisepticum (MG) Infecti....pdf
https://www.freedomrangerhatchery.com/blog/protecting-your-flock-what-you-need-to-know-about-mg-ms/
https://afarmgirlinthemaking.com/respiratory-conditions-poultry-mycoplasma-in-chickens/
https://www.maine.gov/dacf/ahw/animal_health/documents/backyardpoultry/Mycoplasma Factsheet.pdf
I hope this helps!