If it is the case, it probably means that the environment in which your pet lives is infested with fleas. As a matter of fact, and despite common belief, fleas do not permanently live on the animal. Fleas are more often on the floor, in carpets, pillows, etc… Fleas “jump” on the animal only when they need to be fed with blood. The treatments you are using kill fleas that are present on the animal. But if there are a large amount of fleas in the environment, new ones replace dead ones giving the impression the products are not effective. Therefore, THE ENVIRONMENT ALSO NEEDS TO BE TREATED. Once it is treated, you will have to prevent fleas coming back by treating them as soon as they re-appear with a shampoo, a powder, or a spray. Ideally, you will permanently have a collar on your pet. You would need to replace it at the end of its life as described on the packaging; it would permanently protect your pet against fleas.