I have used frontline on my dogs for many years. I raised and breed many Jack Russell Terriers. Nothing handled fleas like Frontline.
My oldest terrier now is 17, in great health. Frontline has become less effective to zilch lately. Due to his age I am very careful. He has always had been sensitive to fleas, frontline has always worked until lately.
Maybe try a new product such as advanatge???. vetcare