I would separate this hen away from your other birds for treatment/observation.
It's possible she got dust in her nares and is just sneezing... Is it constant or isolated events? Do you see it happening mostly when she eats?
She could have the beginning of a respiratory infection. Treatment will depend on what the infection is... There are drastically different options available for Infectious Bronchitis versus Mycoplasma, for example.
Could you take this bird to get a blood test at your vet? Is she the only one showing these symptoms?
I would isolate her and check her poops and look her over for mites and lice. When was the last time you wormed your birds? Parasitic load can knock a bird's immune system back and let a smaller infection take hold.
Let us know!
MrsB
