It is very hard to explain pollinator populations when it seems like they suddenly disappear from a local area and/or are very abundant in another area. In your case, it is probably just a local occurrence. I have seen a fairly robust pollinator population this year on the native flowering plants in Leelanau County where my office is located. One thing that I often recommend is to watch the plants throughout the day and to be sure to check them at different times of day. Pollinators sometimes forage earlier or later in the day to avoid the heat at mid-day. Also, sometimes the plants are foraged so heavily in the morning that they are actually completely tapped of nectar and devoid of pollen by mid-day. They can usually replenish the nectar supply by later in the evening and sometimes you will see bees return to take advantage.