My Lady has constant uti's because of her diabetes so I am sort of an expert!
Did they culture her urine? They have to insert a tube and get a sterile sample, then send it to the lab. It's takes several days to get the results. Only then will they know what particular bacteria is causing the infection and which antibiotics to prescribe.
Lots of times your vet will just prescribe Clavamox as it takes care of most uti's. If that doesn't work, most likely you need a different antibiotic. The majority of uti's in both humans and dogs are caused by E-coli which has become very resistant to antibiotics. Lady had a nasty one this winter that she couldn't get rid of. It turned out it was two different strains of E-coli so she needed a combination of antibiotics.
Did your vet take xrays to make sure she didn't have stones? Check for crystals?
If your vet has done all that, you may need a specialist. If not, I think your vet can probably handle it. He just needs to do more tests.