I"m sorry that she is still not improving.
If you suspect that she has ILT which is a
herpesvirus then antibiotics would not have been that effective and only treated secondary infection.
It would be a good idea to see if you can get some swabs tested to find out what you are dealing with then you can give the correct treatment if that's possible.
Testing can be performed through your state lab system - you can find that here:
http://www.metzerfarms.com/PoultryLabs.cfm
Or you can send to an independent lab like this :
http://www.zoologix.com/
She is still housed with the other chickens and none have had symptom or they got better quickly with treatment? What are you long term goals with keeping chickens?
https://www.merckvetmanual.com/poul...ew-of-infectious-laryngotracheitis-in-poultry