Did they run a culture to see what bacteria it has? Some antibiotics only kill certain germs. With the culture they grow the bacteria in the lab and then test which antibiotic kills it. I had a snake once with a RI and they started him on an antibiotic thats kills most infections but we still ran a culture to be sure. When the results can back days later we found that this particular bacteria needed to be treated with a different antibiotic that could only be injected, which I had to do daily, but she recovered fine.