After Calleigh tells Natalia about the distinct marks on the arrow from the launcher of the bow, Natalia takes the bow and walks away. When she takes the bow, she isn't wearing gloves and would have contaminated any evidence on it.
Calleigh uses distinctive marks on an arrow to find out which bow was used to fire the fatal shot. Considering that the arrows used were from a high school shooting club, where the arrows were available to all members, the arrows should have had marks from two or more bows on all of them. Thus unless the fatal arrow had never been used before, and only have gotten marked by one bow, there would have been no way to match it to a particular bow.