If it's the same arrows ?Spine dose that some time. If the arrow is under spine for your weight It bend more and loose energy. The stiffer it is the less energy is lost. If you shoot over a chronograph and put a 1/2 a turn down on you limb and shoot again and repeat this you will reach a point were the arrow will start slowing down.When doing thiswith a chronograph andfinding the fasted point the arrow shoots at is what we call the sweat spot and the arrow will perform at it best there. When getting a new size of arrow tune the arrowto the bow to get the best performance out of your set up. You can do this by group tuning to by adding and taking off poundage. When the arrow working at it's bestyou will shoot your best groups.