Just curious but are you using the regular Frontline, or the Frontline plus? I have also heard that there have been some cases of fake frontline, (esp on ebay) where it is nothing but vegetable oil, but no idea if that is true.
Been using the Frontline Plus. Used to get it from the vet, but now getting it from the feedstore. in between I was getting it from a local hardware store. Tried several outlets thinking maybe I'd gotten hold of a bad or old batch. Its really frustrating, since I'd gotten used to putting the stuff on them once a month and not woryying with fleas on them or in the house again. 
Have you considered contacting Frontline to see what they have to say?
Maybe it is a quality control issue or maybe it is something that they need to be made aware of.
Fleas stink!!!
