Maybe you are making the steps to big(I don't mean your paces

)
What I mean is: when you taught her to back up, you probably stood still, or you moved towards her.
And now you are backing away from her. That is confusing.
What I would do is:
Give back up cue-as soon as she backs up, I take a tiny step back. Reward even if she only moves one or two steps. I'd repeat that untill she moves back like she did when I stand still.
Then I would start making my step larger, untill it is a normal step and so on.
Another thing you could do, is to teach her to back up to a target.
That should make it easier for her to back up, while you move away.
But if you haven't trained back up to a target, then you would have to re-teach the trick.
I have been thinking about this too, as I am not using a target. But I am not very good at fading the target, so I am still thinking about this
