I've not heard of soaking sunflower seeds. Lots of others, but not those. Maybe that's it?

Most likely reason is something is eating them. Voles, mice, chipmunks, squirrels, birds all love them and will eat them every opportunity. You may want to either try coating the seeds with something distasteful such as Captan fungicide and dusting a bit over each planting hole, or start the seeds indoors in cell packs, grow the seedlings on outside, and transplant in-ground when they have 2 pairs of true leaves.

I soak my seeds to ensure better germination, but I don't plant until they are at least 4 in or > in height. The birds will swoop down and eat them if planted when very small.