Before anything like some previous posters have said, really make sure she is on a correct lead when approaching anything at the lope. If you are able to rule out any issue with teeth, chiro or tack fitting then there are some things you could try.
If she is just being a pill and won't lope into her first, start at the end of the arena before making your approach to the first barrel and lope some good size circles, making sure to circle the SAME way you are are going to turn your first barrel. Lope a few to make sure her body is collected and she is listening to your cues, then come right out of the circle and start into your first barrel that way to get the lope. Make sure your approach/pocket is giving her enough space to make a good turn.