Quite often there is only a small part of any given day when pollen or nectar are available from different flowering plants. For instance, buckwheat will produce nectar for maybe only a cpl of hrs and then shut off. It seems to really irratate the bees. One doesn't want to work hives after the buckwheat shuts off. Fortunately, or maybe not, there aren't very many strains of buckwheat that produce nectar anymore.