I know that the butterfly bush flowers will vary in the amount of nectar they produce. Maybe they need to probe more flowers to get the same amount of nectar. My Monarchs spend less than a minute nectaring on the gatorade before they fly up, too. I do look at their abdomens after feeding and their abdomens are plump. If a butterfly isn't getting enough water or food, their abdomens will shrink.
Herndon, VA (USA)
Take care of the small things....