One of the most common mistakes for novice seed savers is to not allow the seed to mature thoroughly before harvesting. Although seeds may continue to dry and harden, seeds don't really continue to mature after the seed heads have been cut from the plants. B. vulgaris seeds should have turned from green to tannish-brown before being collected. The spikes mature sequentially from the base of the flower stalks upward.
Photo courtesy Seed Savers Exchange