If you are working in the grazing place, some horses get a bit narky, seeing it as their territory and not a work place
If you have an area out of the field, maybe that would help too.
Could you get someone to ride her on the lunge?
If she is unsure about what the lunge is, it may help to get her used to the line, the circling, the whip etc.
It helped my horse who was as thick as a plank when it came to lungeing, now at least he will go properly (with the occasional bit of...