Bests & Worsts Reviews from Amazon

I live in California and I've used K9 Advantix for the entire life of my 6 year old Schnauzer, except for a very short period where we switched to the Kirkland brand. After fleas and many ticks were found, we returned it and stuck with this because worked so well. I've even bought it for the past 4 years from Amazon and same seller, but having started this new box, he suddenly has fleas and ticks again! I washed him with Dawn and tried it a couple more times, in case we had made an application mistake- still had fleas! I even went to PetSmart and got a different box, thinking that it must have been a bad/old batch. It's been a week now and I just found two live fleas on him and he's scratching like crazy! I compared lot numbers on the two boxes, just in case, by some fluke, they were the same. Completely different! This formula just doesn't seem to be working anymore! I guess I return the stuff to the pet store (too late to return the first box to Amazon) and get Nexgard from the vet... UPDATE: When I went to the vet, he that they had been seeing this more and more in our area. (Northern California) Because we are surrounded by agricultural, and exposure to pesticides that are similar in composition to these products, the pests become resistant faster. So if you are in a very urban area, it might still work for you.