My experience with treatments like Frontline is they're only effective if used before there's a problem. I use it when the weather starts turning warm and then stop once winter rolls around, and that keep fleas in check in my area.
I agree with the others, it sounds like you might have an infestation inside the house or wherever your dog sleeps, and if that's the case the entire area needs to be treated to wipe out the flea population. If you really aren't seeing too much evidence of fleas in the environment, it might be something else in conjunction with the fleas that's causing the itching. One of my old dogs had severe skin issues that flared up all the time, so even a couple of fleas would cause him to break out in rashes. In my case the vets never figured out what was wrong with him, but we mostly managed his condition with diet and vigilant grooming.
I agree with the others, it sounds like you might have an infestation inside the house or wherever your dog sleeps, and if that's the case the entire area needs to be treated to wipe out the flea population. If you really aren't seeing too much evidence of fleas in the environment, it might be something else in conjunction with the fleas that's causing the itching. One of my old dogs had severe skin issues that flared up all the time, so even a couple of fleas would cause him to break out in rashes. In my case the vets never figured out what was wrong with him, but we mostly managed his condition with diet and vigilant grooming.