Is it moving a lot in her month? I know when I first was green breaking my horse, he would open his month and let the bit slid right out, off to one side or another. We ended up putting a full check snaffle bit on him. He even did it with it fitting right in his mouth. He also use to shack his head and play with the bit when he was bored. I thought about getting a bit that had rollers on it. So he would have something else to play with.
I have also seen people take new bits that a horse may not like and tie baler twine to one side of the bit and then put the bit in the horses mouth and then take the twine and put it around the horses poll and tie up the other side of the bit, so it was like a homemade headstall, and leave it in the horses mouth for an hour or so a day while they are in there stalls so they get use to the bit. To some people it may seem mean, but I does work. If you choose to go this route just make sure that the only thing avalible is water, as horses should not eat hay, grass, grain or anything else while having a bit in there mouth. Hope this helps.