Just a suspicion....Like most beginners, she thinks that you have to really, really, really, get upside down to do a back handspring, and really try even harder to get upside down for a round-off back handspring. When she lands her round-off she is "standing" there doing little to nothing with her legs to move into the back handspring, and is putting all of her enrgy into "arching" around and "reaching" with her chin. Either of these (arching/reaching) can cause her "strong side" back muscles to dominate the flight and snapdown phases of the skill. If it's not that, I'd consider really looking at her tumbling from an end on perspective to see if she is going in a strait line and over the top with her feet, and no arch through the handstand phase. Help her fix anything in her round-off, and her bhs will be a lot easier.
There's a post from about a month ago under the title "round-off problems" where you might find some ideas tosolve "balance" and direction issues. The information is kind of "scattered", probably because the post was done at 1:37 am, but the ideas are pretty good if you can decipher them.
