My neighbor has them at her house-her 16yr old dog is allergic to fleas which makes it worse for her. They have tried every flea product but she is still finding fleas on her.
I live right next door and i have never ever seen a flea on either of my dogs-i check them everynight at brush time-i hate the thought of fleas!
I did do a little research for my neighbor and some shampoos can lessen the affects of some flea treatments, so maybe try a natural gentle one. Also you can make a rosemary rinse (its a natural flea repellent)-just put some fresh rosemary into some water and let steep on the stove for 10 min let cool, after shampooing pour over dog make sure you cover the whole dog and dont rinse.
Also as others have suggested, get an exterminator in to do the outside and in.
Hope this has helped.