Your package will have an expiration date on it. It may have sat on a store shelf for years before you bought it.
According to this vet http://www.thecatsite.com/t/7765/shelf-life-of-advantage-flea-control, 5 years.
Maybe you have an infestation in your house somewhere and they (and you) keep getting re-infested. You might have a pest control company come out and check/treat your house.