Is it happening overnight? Perhaps it's birds or mice. Even squirrels. Fire ants can also eat seeds.
If you suspect ants, sprinkle a little Sevin.
If you suspect mice, spritz peppermint/cinnamon based mouse repellant around. Such as Rodent Sheriff.
As for squirrels...use a gun.
For birds, try tying balloons or another type of scarecrow out.
I hope you can figure what's going on.
Pests in the garden is such a pain.