My vet usually does a cystocentesis in order to get a sterile specimen if bladder problems continue. It could be that the antibiotic has not worked, or that it is not the correct one.
There is another medication, Prazosin, which is used for various bladder issues in cats. It is not an antibiotic, however, and is more of a sedative/relaxant that makes urination easier. I have used it for FIC and had it compounded into chicken flavor which was, oddly enough, a big hit. You would have to ask your vet if this might have any place in treatment for your cat or not as we are not vets.
There is another medication, Prazosin, which is used for various bladder issues in cats. It is not an antibiotic, however, and is more of a sedative/relaxant that makes urination easier. I have used it for FIC and had it compounded into chicken flavor which was, oddly enough, a big hit. You would have to ask your vet if this might have any place in treatment for your cat or not as we are not vets.