Weird. It could be that one dog was more "tasty" than the other, or maybe it was an application issue this go around with the one dog.
I've always found Advantix to be effective. My Bat-dog is quite tasty to ticks and she will collect them while her brother Moose-dog is spotless. The Advantix has always kept the ticks off her.
I had a problem with Frontline and fleas way back when it first came out - in those days, it didn't actually kill adult fleas, only made them infertile so they couldn't breed which didn't work if fleas were coming in from neighboring yards and pets - so I've never switched.
I guess this just validates the argument "if it ain't broke don't fix it. Stick with what works for you. :)