To a budding (pun intended) gardener, nothing is more frustrating than getting heirloom seeds, only to wait weeks for no results. Sometimes you just get some bad quality seeds that will never sprout no matter what you do, but assuming the seeds are live and able to germinate, and you have no disease or pest problem, here are a few things that might be going wrong.
1. It's Too Cold-- or Too Hot
Each specific seed type will have temperature requirements for germination. Some seeds do very well inside, and in room temperature soil. Some need warmer soil and more light, and some require darker conditions and cooler temperatures, due to typical planting taking place later in the year. A soil thermometer will work wonders in telling whether your temperature is ideal.
2. The Seed Was Sown Too Deep
A seedling can use up a lot of it's growing energy in moving upward. The leaves will want to start getting light as soon as possible to begin photosynthesis, and if the seed is too deep underground, it might be a long time before that happens! Every seed is different, but make sure you bury it at an appropriate depth. This can also aid with light and temperature issues, as the deeper the seed grows, the cooler and darker it is.
3. It's Over or Underwatered
Once again, this depends on the seed in question. Some varieties need a small amount of water, and can be germinated in a damp paper towel. Others require a very wet environment. As a general rule, you want the soil to be moist-- not wet-- to the touch. A growing seed and emerging seedling need plenty of water, more proportionately than many mature plants, so make sure they get it!