>he said that it had to be a
particular type of buckwheat, i.e., Japanese.

I had not heard the bees would only work one Varity but have little
experience with Buckwheat (a few bags at my wildflower location) although
quite a bit of experience with Heartsease (same family).

Possible answers:

 My guess the weather, humidity and soil conditions played a roll.

To get bees to focus on a new plant they need to be moved in when the plant
is in full bloom. If not bees continue to work what the have been working.

If you keep your bees in two deeps the Buckwheat honey  might have produced
and the honey went into your deep boxes instead of your supers.

Were your hives at peak strength? Are mites and disease a problem? Are they
queenright?

You must remember that you will not usually see a bee on a bloom unless the
nectar is flowing or they need pollen.

You need to check many times a day  to be sure the bees are not working the
Buckwheat. Buckwheat is very dependable I have been told as is heartsease.

Sincerely,
Bob Harrison

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