My guess is you are stopping your swing, a very common thing with pretty much anyone who doesn't regularly shoot shotguns. There really isn't that terribly much lead required for a muzzleloader ball at reasonable ranges. At 50 yards, a .490 ball with a MV of 1450 fps will take about 0.122 seconds to reach the target. If we assume a deer is moving 25 mph, which I've clocked them doing next to the truck, and that is booking it, then the lead required would be 4 1/2 feet. Unless you are shooting deer in an open field at a full sprint, more realistically a running deer is probably doing 15 mph. At that speed, lead required would be 2.7 feet. This is inline with what I've seen in the field, where based on instinct I'll hold just in front of the chest, and that centers the lungs.