I am thinking that if your cat keeps getting fleas, there must be a flea infestation somewhere that they go. I had a friend whos cats kept getting fleas, even after the monthly treatments. His vet told him to check around the house (inside) since they never went outside, she told him there had to be a bed of them somewhere in his house and he needed to find that and get rid of that bed of them.. He swore he didn't have any fleas in his house but looked, it took him several weeks but he did find it. It was just a old blanket he had thrown in a corner and forgot about and he said it was plum loaded with fleas... It took him some work to get rid of the fleas but he eventually did and the problem was solved.