I suspect that we have to take some care in how we're interpreting the behaviour of the bees when compared with what we know about the plant. It may be a huge nectar source, but does that necessarily mean that bees will visit it in preference to something else? The impression I have from my reading is that bees may do something like a cost/benefit analysis on sources of forage, so if there are flowers that produce less nectar but are significantly "less costly" to visit then they may prefer those over a nectar-rich flower that requires more work (because it's further away, perhaps). This might lead beekeepers to believe that bees aren't interested in a particular source of forage when they certainly would be if it weren't for the availability of one they consider better.
(And of course there's always the possibility that they just didn't find the better source of forage, though I'm not sure how likely that is.)
James