hmmm..done a lot of reading and looking. There may well be an FL resistance(there was talk that Advantage had a problem a couple ,of years ago) but I am wondering if the problem is not the way FL works, in that it's too "gentle", allowing the fleas to exist on the animal, and unless applied very often, to breed. So in really heavy infestation years, all the treatment of your house and yard will not help (especially if it's not residual), if any animal crosses the yard some time after the treatment and drops fleas from outside, it starts again. Many people said that it seemed to work, then it was back again after a few days. One writer said the FL stopped working one year and they switched to something else for a couple of years, then Fl worked again. So that could either be resistance, or FL simply not being good enough in a really bad year.
One thing I am finding most annoying is the FL and vets' attitude to simply deny there could be a problem, and statements being made that the Fl you get online or at non-vet outlets is somehow different (inferior) from the one the vets sell. AS far as I can tell, that was after FL lost its protected status as a drug. but what I buy from the farm shop has exactly the same active ingredient, in the same strength. I guess the delivery medium could be different, but that seems highly unlikely.