Is she the only one with symptoms? How large is your flock - the one she was a part of until you isolated her from the others? Is the nasal discharge smelly? Chances are she has a mycoplasma or chronic respiratory disease. If she has it, chances are high that the others have been exposed.
Some birds are just naturally more resistant to infections than others. Those that lack resistance should not be used as breeders. Stress brings out symptoms, so make sure your birds are not stressed by overcrowding, too cold, too hot, etc.
If this bird is not irreplaceable you might cull her now, but I doubt it will make much difference to the rest of your flock.