to correctly stop a horse, you've got to disengage their hind-quarters.....simply pulling back on the reins will NOT do that, instead it normally keeps their hindquarters (their "engine") engaged.
Try doing a one rein stop next time, that will disengage her hindquarters and make her stop.
Make sure also when you ask her to stop you stop moving. Sit down deeply and put more weight on her hindquarters.
I normally stop my horse with a one-rein stop. It disengages their hind-quarters and also their mind. I'll ask for him to stop first by stopping my movement and putting my weight back, and if he won't listen, I'll do a one rein stop