Are you using the spray or the droplet stuff? It's been a while since we've had to use it (we had the yard sprayed by a pest company after the last feral tenants) but I'm pretty sure I remember it's in the instructions that you can't apply it on the day you wash your dog. You have to wait a full 24 or 48 hours after washing otherwise it doesn't soak into their skin properly. Or something like that.

Also, check exactly how much your dog weighs as you may be giving the wrong dose.

It might help to read a bit more about how the product works. If a new flea jumps on, it doesn't die immediately, but after a few hours. This means that at the beginning, you need to make sure you keep treatment up, and clean/spray all the doggy hangout areas, and keep treating over time. If you're regular about it you will notice an improvement over time.