mobowhunter is correct, centershot is out of whack most likely. This is what is called walkback tuning. Start at say 10yds, shoot an honest 3 shot group at the bullseye. Do not worry that you'll hit above where you aimed.Mark the center of that group. Now, step back to say 30 yards. Shoot another 3 shoot group using the same pin you used to shoot the 10yd group. They will hit low, but what your looking for is whether or not this group is centered with the previous group or left or right. This will tell you which way to move your rest to get them inline. Don't worry about moving your 10yd group. The change will be very minimal at this distance. Once you are as close as you can at 30, trying continuing to move back. I shoot all the way back to 60yds. Of course I'm not shooting at game at those distances, but it helps really fine tune center shot.

Of course, human intervention into the shooting system will also have an effect. If you torque the grip, especially inconsistantly, moving the rest will not help.