Seresto collars a waste of money

I bought 4 collars, two cats and two dogs. Previous time I bought it, it worked which is why I bough again. Now it is worthless. My dogs and cats have flees and are scratching all the time. We live in town and not on a farm, which is where I might have forgiven Bayer. The collars are tight and my animals have been wearing it for more than a month with no results. Do NOT buy this product.