If it is the spot on stuff for the back of the neck, this usually works by killing any eggs that the fleas leave, so they don't hatch out. They don't tend to kill the fleas they have on them, or prevent them getting on them.
I would shampoo your dog with a flea shampoo (or any sort if you can't get any) and get Frontline spray from your vet, which is a 'knockdown' treatment, killing the fleas straight away. This shoud last up to 3 months, but can be done again in about four weeks, if the animal is in contact with lots of fleas (i.e. catches a hedgehog or runs about where there are wild rabbits, or cats sleep in your garden).
Personally I don't like the spot on stuff even though I sell it (some people want a cheap version on the market) but I believe its a waste of time. I'd rather spend out on a bottle of Frontline which will last for several 'sessions' with one dog.
Make sure you shampoo the dog first, to make sure the old stuff is out of his coat.