I see bees working flowers early in the morning, then along towards late afternoon the bees don't seem to be working the flowers. My question has to do with how soon the plant replenishes the flower with nectar? I suppose after a good rain the plant absorbs the moisture through the root system providing an abundance of nectar, where as just the opposite occurs when no rain has fallen for several days. But on average, do you suppose that once a honey bee has depleted the supply of nectar in a flower that it's done providing nectar for the remainder of the day? Do you suppose nightfall allows the plant to replenish the nectar supply in the flower for the next day? A question for a horticulturist I guess.
Some plants only secrete nectar during certain periods of the day. (Controlling factors are primarily temps, though I suspect day length is also a less-important factor.) I think that's why people are sometimes confounded when a plant that reportedly is good nectar producer doesn't seem to be getting much action. Perhaps at the point you're observing it, the nectar flow is over (or hasn't begun in some cases) for the day.
You are also correct that during periods when rainfall is unusually scarce for the area and the season that there is reduced nectar production. Conversely in periods of unusually heavy rain the nectar can be washed out, either for the season, or for a day or two. (It can also become diluted, and thus less attractive to those efficiency experts: honeybees.)
And lack of rainfall during certain growth periods will later reduce the nectar available months down the road (or even the following season in the case of woody plants), because it reduces the amount and size of the flower clusters the plant later develops.
Bees have co-evolved with many nectar plants, and differing nectar production periods during the foraging day ensures that all of the plants are equally exposed to the services of bees, which after all are essential for many plants' reproductive success. Nectar being just the inducement offered to the bees to visit and pollinate the plants.
Quite a remarkable system, when you think about it.