There are other medicines that may treat your bird that is not recovering with just the med you've used. There is a Poultry Medicine Chart on my site linked in my sig where you can look up some possible options.
I wouldn't give the same med any longer, as it may be killing off too much of particular "good" bacteria in your birds too much & also may be hard on their systems in other ways. You can give tetracyclines (the class of med Tetroxy is in) longer than many other meds, but more than 14 days would be really a long time.
I would try switching to another applicable medicine that may finish off the bacteria that are particularly resilient in your one bird. If you are able to give the new med to just the one sick bird, that may be ideal if it is one you can administer easily enough to just one.
It would also help all your birds to get them eating probiotics to help rebuild their good bacteria now. Personally, I even recommend giving it to birds sometimes during medicine treatments if you can stagger it to be several hours away from when you dose with antibiotics. That way the good bacteria can fight bad germs for at least a few hours every day.
Your one bird may gradually recover without more medicine, or she may end up with a lasting problem that can't be cured well.
