Many pesticides are active compounds (i.e. they contain either oxygen or nitrogen atoms within their structure) and as such, you can experience tailing, or peak absorbtion, when analyzing them by GC. Ensuring that your GC liner is deactivated, and clean, are good first steps. Using a gold seal s is also good practice, and changing the gold seal every second liner may well be required.
Finally, the choice of a highly deactivated GC column will greatly aid you in getting good peak shapes and responses. For the analysis of pesticides we would recommend the ZB-Semivolatiles column to provide the best peak asymmetry values and limits of detection.