Do you have a cat ? I think they are the prime candidates for bringing fleas into the home as they get them of all the creatures they catch! Or hedgehogs in the garden ? Is she a rabbit catcher ?
A friend of mine found fleas on her 10 wk old puppy, yet she had had a frontline treatment. Her adult dog was scratching but vet found no sign of fleas on her! I suggested that it may have been the cats fault, maybe bought something in with fleas, or that puppy found a hedgehog in the garden!
Hope you find a solution.