Make sure you are putting down the right type of grass seed and not just cheap contractor annual rye or something. Sand is not a top dressing material. Something like peat moss, straw (not hay--it has too many seeds of its own) or a redwood, etc. mulch would be a better idea.
Did you prep the bare spots at all? Hit and miss if you just sprinkle seed on top. And remember you have to keep it moist until it germinates. Then adapt to a regular schedule aiming to water deeper and less frequently.
Crappy Annual Rye - 5-7 days
Perrenial Rye - 7-10 days
Bluegrasses, Fescues, etc. - 14-21