When conditions are right fleas in the surrounding enviroment can be so thick that they provide constant replacement for the ones your treatments kill. Giving the dogs the appearence of one continuous infestation. Before turning you dogs coat into a toxic waste site take the battle outside and kill the fleas and their eggs where they start. Since they took away Diazinon the best bet seems to be a Bifenthrin based insecticide like Ortho Bug B Gone Max. For inside I have found that treating the dogs bedding and carpet areas with Permethrin based flea powders kill them where they hide before our four legged bug mops can pick em up.
If there are no dogs in Heaven, then when I die I want to go where they went.