Campy and Geistware covered most of the possibilities. My guess would be that you don't have the servo arms centered on the servo. Take the arms off, center the servos, and reinstall the arms. When you center the servos make sure all the possibilities are centered. This means that you travel is centered, sub-trims are centered, End Point Adjustments are set to 100%, trims are centered, exponentials are removed, and any mixes applied to that channel are removed.

Hope this helps.