I've heard that fleas can become resistant to Frontline, though I have not experienced that myself. I would suggest trying another product like Advantage or Advantix. Your vet can probably recommend a few others to try.
Make sure you're using something with an insect growth regulator to sterilize the fleas before they lay eggs. That's really the only way to break the cycle. Are you using regular Frontline or Frontline Plus? Frontline Plus contains an IGR, the regular Frontline does not.