Be sure that you are cutting

Be sure that you are cutting the seed right in the middle, evenly along the seam of the seed coat. Also, make sure that the fruit is ripe; it is possible that the seeds have not fully developed yet (that's our best guess).

If you've cut the seed properly and the fruit is mature, we're stumped. Could it be that it's just the way your plant grows? There will be some variability in seeds depending on cultivar and species of persimmon (American vs. Asian persimmon, for example).