We have used flea meds in the past on our cats and it worked just fine (same brand). However, this is the first time in a year at least that we bought some. After putting it on the backs of their necks, we assumed the fleas would die as in the past. However, now the flea dirt is out of control and there are MORE fleas. What happened? Should I ask my vet for more (free)? We spent for three treatments. Why didn’t it work this time?