It may be because she isn't very flexible through her back/shoulders so just be patient. Try making her reach for her cookie towards her ankle, and the second you feel her go to that place, give her a treat. Do that from both sides to loosen her up a bit. Try not to exceed her abilities too soon, or she won't try. :) So first maybe work on her trusting that it's okay to stretch those muscles, and you'll find little by little she'll stretch a bit further.
The little guy I'm teaching to bow I was thinking he'd never get it. Then all of the sudden one day he offered it and then it was there. So just be patient. :)