The biggest problem most people have with planting any seed is they either plant them too shallow or too deep. Too shallow means the seeds might imbibe water, swell, and begin to germinate only to have the soil lose moisture. This, of course, causes the developing seedling to die because it, too, dries out. Too deep means that the seedling just does not have enough ooomph to make it out of the soil. It uses all of its stored energy reserves trying to elongate and push through the soil only to fail before it can reach sunlight and start photosynthesizing. Crusted or compacted soil aggravates the problem and traps the seedling underground where it dies or succumbs to soil pathogens or pests. We always plant our sunflowers less than 2 inches deep (5 cm) for large seeds, and 1 inch (2.5 cm) for smaller seeds.
There are other factors that determine your planting success. Wireworms and other larvae (commonly called grubs) can attack and eat the young seedling underground before emergence. Look for partially eaten seedlings beneath the soil or seeds that look like they have been drilled out!
Once emerged, your seedlings can be mowed down by cutworms, snails and earwigs, for example. Insects like sprouts, too. We use a bit of insect bait around the planting area just to be sure. Finally, watch for birds. When the sunflower seedlings are pushing through the soil, the hulls frequently adhere to the young plants. A bird sees the hull as a seed waiting to be eaten. They peck the seeds and break the stems of the seedling. You might try a length of wire mesh, bent at a right angle and placed over your planting area. Rarely do birds attack sunflower seedlings once they are established.