Mare won't back up..?

from the ground, put pressure on her chest and say back up and when she steps back stop pressing, keep doing this and she will learn the command, then when you try it under saddle she will understand better what your asking her to do. it sounds like shes never done it before so shes just feeling you shift your weight and pull on the reins and shes thinking what are you doing? and just putting her head up because she doesnt know what else to do.