Is this just a bad flea season or are the fleas immune to Frontline Plus?!?!?

Three of my four kitties are indoor cats. They NEVER go out. Only one goes out and, most of the time, she's out for an hour or two and then crying to come back in.

Anyway, I treated all four with Frontline Plus because the one outdoor cat brings home fleas. Well, we STILL have fleas. Not as bad as it was, but we still have them.

I asked the vet about fogging the house, but he doesn't recommend it. He said to just use the Frontline Plus and to vaccuum every day and toss the vaccuum bag. (I'm the only one getting eaten alive by the fleas. My husband and daughter don't get bitten at all!)

Is anyone else having the same problem???????