I had a doberman that had chronic bronchitis throughout her life. If the antibiotics are not helping, see if the vet will give you a little prednisone. My dobie used to take prednisone with clindamycin for a few weeks and it would clear up. Then a few weeks to a couple months later, it would return. After a few years, it got to the point she lived on increasing doses of prednisone. I consulted with another vet, we finally discovered it was due to inflammation because of allergies. Some dog foods were less reactive than others, but all seemed to have issues. I started making her food at home and she was able to come off meds and live the last few years of her life symptom free. Obviously I'm not a vet, but wanted to give input as I can relate to the frustration of it. Also, all vets and research have said that allergy testing is not the least bit accurate so it really comes down to trial and error. Good luck.