Buckwheat will continue to bloom until it reaches it's full height. It grows in spurts and will bloom, seed, bloom, seed, etc. This cycle goes on for about 10 weeks. Depending upon when you planted it you may be writing it off too soon. Bees will usually work buckwheat in the morning, and late evening hours as this is when it produces most of it's nectar.
As for something else, there's not a whole lot to choose from unless you like white dutch clover. It will normally come up and flower first year as long as you get enough rain. It is also controlled by the rain as to how much it produces. The nice thing about it is once you get a good stand of it, say a few acres, you mow it just prior to a good rain to clear the dead heads, and then two days later you have more blooms for the bees to work.
If you want to think ahead to next year, white or yellow sweet clover will do the trick. Will come up this year, and not flower much at all, however next year it will regrow from it's root and put out lots of bloom.