I've never had that problem, but, I haven't lived anywhere that ticks were a major problem. I don't know if maybe not all ticks are covered in the protection? I'm just brainstorming....I know there are different kinds of ticks. Ummm....maybe check and make sure you're using the correct dosage for the weight of the dog.....and, that the medication is going onto the skin.....is your dog able to lick it off? Again, not assuming anything, just brainstorming!