Why does this stuff not work?

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All 4 of the dogs got their tick medicine put on them a few weeks ago. It seems to be working on 3 out of the 4 dogs. The one dog is like a tick magnet. Even after his second dose coming early at the vets suggestion. I have gotten no less than 40 ticks off of him in the last 3 weeks alone. And I am talking attached, not just on him. Why does this stuff not work? Have you ever had this problem with a pet? Or any other medicine that just does not work?