If your horse is reluctant to flex to one side when being ridden it could be behavioural or a lack of training. However if your horse has previously been willing and able to flex to both sides and this suddenly changes it is possible that there is pain causing the resistance. Pain anywhere from the mouth all the way along the spine can result in a reluctance to flex. Your veterinarian will be able to diagnose dental issues and pain arising from skeletal or muscular origins. One example of this could be overriding of the spinous processes of the thoracic vertebrae causing pain. This can be exacerbated when riding and potentially when the horse is asked to bend and flex to one direction. If you suspect pain as the cause of your horse’s flexion avoidance you should call your veterinarian.