Anyone else experience a complete failure of Frontline Plus for their cats?

[Flaxbee]

I put a full dose on each cat about 2 weeks ago and the poor kitties are really struggling. Fleas fleas fleas. As if the Frontline Plus had no effect -- the last time I used it, the cats were fine. This time - about 5-6 weeks after the last Frontline dose, the cats still have fleas, and not just one or two. We had a really very hot few days here, and I know heat can make things worse - but not if Frontline is in the cat's system, right?

I have also investigated Revolution, but that product doesn't seem to protect against ticks, unfortunately.

Help!