We had a litter of pups who managed to get fleas. You can't treat puppies for fleas so we did our best to manage the situation with combing etc but you can image how tough it got in those 8 weeks - the stuff nightmares are made of!
We found frontline wasn't very effective. It definitely didn't work on our cat, and when we hired a pest controller he said the same about Frontline. We used oral tablets from the vet - Nexguard - and they worked far better for our cat and dog. We treated the pups with Frontline as soon as we were able but it wasn't very effective unfortunately.
Also, once fleas are in the house they're 30% on your pets and 70% in the environment. Flea eggs can hang around for a year before hatching.
Thankfully flea treatments really helped, and a lot of cleaning. Because our situation was so bad we had the pest controller come out and spray (which was worrying for our pets)
But a couple of fleas found on your dog should be easily treatable if they haven't laid eggs in the house, and once treated any hatching fleas which jump on your dog should die with a day.