http://www.forensicmag.com/article/2012/09/science-behind-gsr-separating-fact-fiction

The absence of GSR does not indicate innocence. There are factors that contribute to particulate loss that must also be taken into consideration. For instance, suppose a firearm is discharged in a static environment (no wind or airflow). Hand samples are then collected immediately after the discharge on clean, dry hands. It would not be unusual to detect hundreds to thousands of GSR particles on these samples. Alternately, suppose a firearm was discharged outside during a windy day with inclement weather. Samples are collected several hours after the discharge, giving the subjects ample time to move, shake off, wipe, or wash their hands. In this instance, it might not be possible to find any GSR particles on the samples. This absence of gunshot residue is not an indication that the subject is innocent of discharging a firearm. All factors concerning particulate loss must be taken into account when determining if results are consistent with the aspects of a specific scenario.