Did this chick come from the same place as the others?
Hard to know the cause. Where are you located in the world? Has the chick been outside/exposed to mosquitoes?
I would still lean toward respiratory disease you can try an antibiotic along with the daily flushing of the eye/applying the ointment to see if that makes a difference.
https://www.backyardchickens.com/threads/any-idea.1395780/post-22913348
If they all came from the same place and have lived together, whatever this is, likely the others have been exposed. I would still keep this one separated for treatment and as mentioned in the thread - wash your hands/change clothes between handling this one and the others to help minimize spread.
https://edis.ifas.ufl.edu/ps044