I sow mine at a half-inch. If they haven't popped by the fifth day then I'll carefully pull back just enough soil to expose the seed. What usually happens is the root is growing downward while the seed head (not sure of the correct name, but you get the idea) gets stuck under a heavy clump of soil. You want to avoid messing with the root and just make sure the seed is able to lift out of the soil on it's own.
Also, when germinating, I find that its best to use a potting soil specifically formulated for seed germination, like Fox Farm Light Warrior or Miracle Gro Seed Starting Mix. You can even use pure coir. However, regular potting soils are to strong and can burn the seedling.
And sometimes, even seeds that come from the best of stock, aren't viable. Its not always your fault or bad breeding practices. Just bad luck.