I have purchased Seresto collars for my dog and cats in the past with no difficulties...

I have purchased Seresto collars for my dog and cats in the past with no difficulties. This year, they are not working. I purchased a large dog collar for my dog in July when her old collar was due to be replaced. I purchased from PetCo in Barboursville, WV, USA. The collar never worked at all. ZERO protection from fleas. I returned the collar to PetCo in August and they replaced it. The collar worked for about a month and now we are back to having fleas on our dog. I treat my yard and home and her bedding and crate area for fleas as often as prescribed by the insecticide labels. My cat WAS wearing a Seresto large cat collar, but it did not work either. I opted to try him on a topical treatment. I want to know why you've changed your formula or manufacturing process. These were the very best defense against fleas I have ever found. Now they are worthless.