You need to use frontline on the dot every 2 months as you clearly have a problem. Fleas getting on your dog with frontline is normal (living in sheep country, I regularly find a flea wandering around the face of one of mine) - the point is they won't stay or bite the dog. assuming it is working.
As you are using frontline regularly, I'm afraid the problem is your house rather than the dog.
When I moved into my house, before I had dogs, the house was crawling with cat fleas from the cats of the previous owner - I'm a flea magnet and get bitten before any animal
. I got the council to spray the place and had to have the place sprayed again about a year later when a friend came round with her cat fleas
Getting rid of the front room carpet cured it completely.