Is it possible that you are concentrating more on those first 15 shots and then become lax and not concentrating? I know when this happens to me, it can sometimes be credited to the above. Like the above poster indicated, you might try to break away from the norm and go stumping. It seems to help me if I get in a rut.
Also, like Macbow said... you might find yourself taking more time at the shot if you can shoot with a partner.