I have 2 dogs (lhasa apsos- with lots of hair). After reading another thread on here, I found a flea, and then another, on one of them. I checked and I was late giving their regular monthly frontline plus. Then, I noticed that both of the dogs are scratching and acting annoyed when they are trying to nap, so I assume there are more fleas than I have seen. I gave them frontline last night- how long should I wait before assuming that it isn't working? It has always worked on them before as preventative, but I have noticed a lot of people saying that it isn't working anymore. We have a lot of trees in our yard and I hate to give up the protection for ticks.