I would think that you might wonder why she is so stiff. Is there some sort of physical issue why she doesn't bend? And do you mean she won't reach her head around to either side? Will she do this if you hold a carrot or treat off to the side? will she reach around and try to get it?
That might be something to try. There are a whole string of "carrot stretches" that one can do. I don't know them all, but of course, having the horse reach around to the side, having it reach down all the way to the ground, and then farther down, between its' front legs. Use your imagination.
Once you see what she is capable without a human onboard, then you might be inspired to ask more of her.
However, if she will stretch unmounted and not mounted, then there might be something like a poor saddle fit that discourages her from bending.