It usually means you waited too long to plant them. Seeds have a very limited supply of nutrients for the sprout. If none are found when that runs out, the sprouts are history.
They should be planted so that the seed top is JUST under the soil. I sometimes leave it so the seed head is showing it's topmost surface.
The soil has to be FIRMED against the new root so that it can start developing it's herring bone pattern of new roots into the soil.
If you got 6 out of ten, that's an ok ratio. Statistically, you'll have 3 females. Grow them to mature plants and take clones. You'll have about 20-30 clones of known females at that point, and plenty more where those came from.
Good luck man!