First off, I wish you best of luck in irradicating these litttle flea monsters..I simply can not stand them..
I use diam. Earth in and around my yard, key is to apply when it is dry outside, it only works in dry form. It is such a small particle that is penetrates the pores of any bug/insect and suffoctes them, thus it needing to be dry. BUT do not apply while windy, since you might then get it into your lungs, wear a mask to be extra protective.
I have tried all the topical flea treatments known to man, but they simply do not work for us down here in TX, the fleas have gotten immune to them (or so I was told, by a very smart flea..;);))
I now use Comfortis, it is a pill, more costly then topical but it works and it works great.
I have not seen fleas on any of my dogs since I started comfortis last summer, and our yard is frequented by vagrant cats, tons of not so clean squirrels and other roaming critters.
Like Tanza said, it will take a few cycles to kill them off completely.
You probably killed the ones that were alive and happy when you sprayed the yard before your trip, but others have hatched since and is busy laying eggs..don't give up yet.