Nectar Deficiency

The brightest part of my garden at present is a mass of vibrant Portulaca flowers, which do not attract the bees. This tiny (blowfly-sized) earth bumblebee worker gave me a clue. It hovered over several of the flowers, but it didn’t settle on any. I took a closer look at a flower and found that it was dry where you’d expect the nectar to be. I wonder why the flower puts on such a show yet doesn't attract pollinators with a sweet reward.