It may be harder to concentrate at longer ranges because it is harder to see a specific spot on the target. I think the main reason we try to focus on the smallest thing we can see, whether it be a hair, a mark on the bullseye, etc., is to avoid letting our focus wander, whether to the arrow point, antlers, or whatever other distractions there may be. Keep in mind that a person's ability to hold a focus is generally shorter than he might think, and you may be attempting to hold longer than you are able because of your uncertainty about the long shot. Just concentrate on the smallest thing you can see at whatever distance you're shooting, and don't attempt to hold any longer for a long shot than you would for a shorter shot.