Why does my dog remain flea free yet my brother-in-laws dogs and one cat are always infested with them. They claim they use the same anti-flea product as I do. Their dogs have thick coats and I'm wondering if when they apply the Top Spot, flea medication they are getting it on the animal's fur and not the skin. As I said before my dog also uses this product and almost everytime we've visited I've never seen a flea on her. The one time I did find fleas was the time we visited and it had been over a month since the medication had been applied. Only because we were in the middle of a cross contry move at the time and the medication was packed away. So my question is, what are they doing wrong? Why does the medication seem to work so well for me and not them?