I used to do this sort of bow, but it is not a true bow. If you want her to bow, you need to teach her to go down on one leg. I do not know if the way you are having her bow is straining or not, but I would not be surprised if it was; that's why I stopped doing it.
This is how I teach bow now. I use clicker training, so I would recommend looking into that. It's quite effective. To make a long story short, pick a leg in which the horse will bow on. Only pick one leg...because switching back and forth between legs is like teaching a new trick. I do not use treats to encourage the horse to bow. I first teach the horse how to pick up the leg on command. Tap with a whip until he picks up a leg...as soon as he does, click and treat and tell him good boy. Repeat the process until all you need to do his point to the leg and he picks it up. You do not want to use the whip too much, because that will start the horse on learning spanish walk. Don't teach spanish walk unless you know what you're doing. It can be dangerous. Once the horse picks the leg up on command, hold the leg and ask the horse to lean back (as if he were going to bow) I used a halter with the rope set up like reins. When I held the leg, I would gently tug on the "reins" until I felt the horse lean back. When he does, immediately click and treat and let go of the leg. Do not make him do the full bow until he has a general idea of what you want. In order for a horse to properly bow, there must be a lot of trust in your relationship with him. When a horse lays down or bows for you, it is a sign that he trusts you. In the wild, horses down lay down often because of danger (cougars, wolves, etc) They often sleep standing up because they can run away from danger better in that position. A true bow takes a long time to teach. When the horse attempts to bow, leave it at that. Slowly work on getting him to lean more back until his leg is on the ground.
I strongly recommend purchasing a video that teaches the bow in steps. Tricks are fun, but need to be taught correctly.