Bees do not collect nectar from all flower sources. There are only about 300 nectar sources in the US that honeybees collect. Some of this has to do with the taste or acceptance of the nectar and also has to do with where the nectaries are located on the flower. The nectaries are located where it is too hard or it's impossible to get for a honeybee. On these plants, you will see bumblebees getting the nectar because the bumble has a longer tongue (probosis) that a honeybee.
Also the weather has to be right, some plants though in bloom do not produce the nectar, becuase it's not warm enough to flow. There has to be a right balance of temperature and moisture in the soil for the plant to produce nectar. It can be too hot to produce nectar too.