Baytril has been the go-to antibiotic for RI's for so long that many strains of bacteria are resistant to it. It's a bit late now for a culture so all you can really have the vet do is try another antibiotic such as fortaz, which hasn't been in use as long and targets a different spectrum of bacteria, in the hope that it does the trick.
Also my vet suggests not feeding while antibiotics are being administered, as they are very hard on the snake's liver and kidney function, so it's easier on the snake's system if it doesn't have to digest meals while receiving the meds.