For a green horse, or one that hasn't learned to give to leg pressure, you may find that they just don't understand your cues. A good way to practice is to face a fence and cue with your leg against the horse's side. Since you are up against a fence, the horse won't move forward, and eventually will step sideways. Keep your other leg well away from the horse's side so he isn't confused by feeling both legs. You want to leave an "open door" for him to move through on that side. When you do get a response of correct lateral movement, release the pressure with your cue leg to reward the horse for doing the right thing. Keep practicing, moving each direction, until your horse sidepasses easily. Once he learns it well, you can start using leg pressure when moving forward in an open arena, and the horse will better understand your cue to move laterally.