Have you also washed all blankets, and vacuumed your upholstered furniture really well? It's puzzling you still have this issue if the cats are on Frontline continuously; they would tend to pull the fleas out of the environment (because kitties are so warm) and the fleas should die if the cats' treatments are current. It sounds like there are flea eggs somewhere that you haven't hit. You could also try Revolution instead of Frontline.
Hello John...we used Frontline on Jade for years, but I believe she became immune to it, as one year she was covered in fleas anyway (I know now, it was due to our backyard being infested). Do try Revolution instead. This worked much better and very quickly that same day. It's comes in pill form and works from the inside out.
I agree with Annie, you've got some eggs left somewhere. Wash blankets, rugs, etc., again, then sprinkle the D. Earth along baseboards and under furniture. Before I say anymore, how are you currently using the D. Earth? You want to make sure it's Food Grade Organic DE.
Here's the link to a the site I've been reading.
Hey again...sorry, I just zeroed in on the fact, your question is in the title of your post. The link I left will cover that, as well as, any other questions you might have.
Good advice from the others.
If it is not food grade DE, it is still useful for keeping the bugs away.
I sprinkle it outside around the foundation of the house. Helps to keep ants out.