Bees that return to the hive are primarily nectar gatherers and get sprinkled with pollen while gathering the nectar. This "double load" condition can sometimes overload the bee and delay or prevent its return.
Any bee entering a hive loaded with pollen, nectar, propolis, or water, is usually granted entrance to the hive. A "home bee" will walk into the hive unchallenged while a drifting bee will be approached by the guard bees, inspected, and allowed to enter if load. Bees (except drones & queens) not of the hive, arriving empty, will be mugged at the door step.