A flushing bird quartering away is the most common presentation upland wingshooters experience. Hitting these birds is easier if you focus your eyes just off the leading wingtip and trigger your shot as you swing from the butt of the bird to that point.
Assuming you have a smooth mount and keep your eyes focused on the target, rather than on the bead, there are two common reasons you’ll miss. Either you’ll focus on the body of the bird and shoot behind it, or you’ll overestimate the lead and pull way off the target. Just a touch of lead with a smooth swing will put these birds in your game bag.