I am having the same problem with everything I use. With 6 adult dogs and 4 pups + living in the country = fleas. It seems that just when I get them under control I start seeing fleas at week 3. I use Frontline, Advantage, Trifexis and recently tried Sentry which I absolutely hate. My vet started my Yorkie out on Combo Guard and same thing. And anything that goes on their backs is a waste of money since bathing literally removes it.
We also spray our immediate yard. My Yorkie, Chihuahua and puppies never go outside so they have to be picking up the fleas on the inside. I'm guessing my indoor/outdoor dogs are bringing them in. It's not like my house is infested but even 1 flea grosses me out.
Just part of living in Texas I guess???