There is more to cocking your crossbow then just pulling it back until it latches. If you want repeatable accuracy it is vital that your crossbow string is latched at the exact same place for every shot, preferably the center. If you are cocking by hand it is easy to vary the latching point. Most people have a "strong" arm which overpulls compared to their other arm and places the string off center. Cocking aids can also place the string off center if they are not centered at the start of the pull cycle. I have seen variations of up to a foot at twenty yards caused by careless cocking. To prevent this problem center the string with your hands or the claw of the cocking aid and keep it centered throughout the draw cycle. The string can be marked with a permanent marker on either side of the barrel at rest. These marks are then used as visual aids for determining center when cocked. If you have cocked the crossbow and your string appears off center you can pull up on the string as if cocking and carefully work it sideways until it is back to center.