I have used Buckwheat as a cover crop...
Did you roll the ground to get good seed contact... water twice a day... like Martin said... new crop seed....
Seedbed preparation
Since the buckwheat plant has a fine root system, preparing a good seedbed is time well invested. The seedbed should be firm to obtain uniform establishment, rapid plant growth, uptake of essential nutrients, to reduce drought injury, and to lessen lodging. Avoid sowing in wet soil because it usually turns out very hard before the root has a chance to grow
Sowing
Buckwheat should be sown with a grain drill at a depth of 1/2 to 1 inch. Seeding deeper than 1 1/2 inches will usually lead to poor and uneven stands. Drilling the seed will produce an even and uniform stand. If a drill is not available, a crop can be obtained by broadcasting the seed at double the usual seeding rate, followed by cultipacking.
The seed should be sown at 40-55 lb/ac. Larger plants will grow on good land, so the lower rate can be used. Higher rates are needed if plant growth is likely to be slow (e.g. if the soil is cold, wet, or poorly prepared at sowing). Large-seeded varieties require slightly higher rates than "common" seed because there are fewer seeds per pound. These seeding rates are based on 85% germination. Germination can decline quickly in common storage, so a germination test is worthwhile if the seed is not from the most recent harvest.
