Where are you getting your Frontline? There are a lot of counterfeit products on the market at the retail level. A member on another forum got counterfeit Frontline from Petedge.
The only safe way to buy flea products are from your vet or a licensed online pharmacy.
Here is some information on counterfeits from the EPA:
EPA: Pesticides - Counterfeit Pesticide Products for Dogs and Cats EPA orders retailers to stop sales of counterfeit flea and tick products for pets - April 1, 2004