Halesia seed is contained in a drupe, so the whole winged "seed" is not just the seed. But nurserymen plant the whole thing with success. Expect them to sprout in the second year. I have examined seed from my Styrax americanus bush/tree, and they are incredibly woody, to the point that I questioned their viability. I have never soaked the seeds, but they do usually float when I put them in water, yet they do sprout when planted. I would suspect that all your seed is good. They sprout quite uniformly, but a month or even two months after most other seeds. So be patient.