V.N.
What brand of flea drops did you buy? With the life cycle of the flea some stages of the life cycle lay dormant for up to 9 months. I am not trying to discourage you. If you are using Frontline + or revolution than you are good as far as the most you can do. It just takes time (3-4 months) to get rid of all the life cycles. The one stage will hatch and you will see fleas and then they will die on your cats. The other thing to do is try to figure out if there is a nest somewhere, that may also be the source of the problem. Wash all of the bedding that the cats use as well.
Good luck. It sounds like you are doing everything right.
Side note: with the topicals (frontline/revolution) they are supposed to be water proof but I still do not bath a day before or a day after application.