I would ask your vet to see if there any treatments you could give them that are better on ticks. I give my dogs frontline, too, and when we go hiking in the woods they get ticks, too, but fortunately since I live in town it's not a problem most of the time. Frontline doesn't seem to be very effective against ticks- one time Marta got a tick embedded in her 1 day after I applied the frontline. They've never had fleas, though.

I don't know how often your dogs are outside, but if nothing else works you could keep them indoors more. Ticks are scary because of the threat of disease. Good luck!