There are a lot of things that can prevent your grass seed from growing, such as the time you put it down, weather conditions, using too much or too little water, or poor soil conditions. It's also important to avoid using products on your grass that could prohibit growth, like weed control products.
Two of the most common reasons for poor grass establishment:
1) Seed was buried too deep or lacked soil contact.
2) Too much to too little water.
Best to broadcast the seed over the desired area and lightly rake the seed into the soil. After seeding, do not bury the seed with a layer of mulch. Yes, the mulch will help hold soil moisture, but it can also prevent the seed from germinating if it is too thick. Keep the soil moist, so seeds do not dry out, until the plants have a well-established root structure.