M-06,
Here's some advicce, a bit of which is mirrored in the latest Am. Hunter magazine.
Try to notice what they do when you pull the trigger. Sometimes they will dive a bit when they see you mount the gun. Try to lead them a little low and they may dive into the pattern. Other times doves that have been hunted will flare their wings and slow down right as you pull the trigger causing you to shoot in front, a situation that a fast follow-up shot can "solve".
A big reason for many missed doves is mis-judged distance. Pattern your gun and see what it looks like at 30/35/40 yds. Also maybe try a shell with a faster load, and different pellet sizes. You may be hitting them with a few pellets, but not enough to knock'em down right away. If you're knocking a few feathers off them, you're hitting them. Just put a few more pellets in the head/neck region and you'll bring more doves. Good luck.