Serious horse problem: How to deal with a horse who refuses to move...but isn't lazy or in pain?

I had an mare that was like this. The trainer I had her at was doing Natural horsemanship with her. And we started riding her in a round pen If you went more than 3 circles in the same gait she would just stop and wouldn't move not even popping her with a lunge whip helped and then we had the same problem when we got into the riding ring (she got so bad she wouldnt even go in a riding ring)so my trainer set up poles and barrels In the ring and we would walk around a few time and then go to one of the obstacles For a minute or two and go back to going around the ring and then we would finish by trail riding her. after a while she learned to bare the thought of the riding ring and round pen because she wanted to go run threw the woods with the other horses.