Teaching my horse to neck rein?

one of my horses knows how to neck rein but occasionally she won't do what i ask immediately or will ignore it and ill end up having to pull the opposite side to get what i want. does anyone know how to make her better at this and to turn when i pull the rein against her. she is quit stubborn sometimes. and i get tired of some trail rides she will ignore my ques and ends up making it more work than fun.