How long ago was the Frontline applied? It can take up to 48 hours for it to kill the ticks. If he had a bath 24 hours either before or after it was applied, or a harsh shampoo was used within a couple days of applying, it might have not absorbed fully before being washed off, hence the not working.

I work at a vet's clinic so I've had to call them with questions about the FL, so this info is straight from them, by the way.