I am a rancher and brought a 5-year old mare back from an outpost where she had been ridden by a ranch hand. He must have inadvertently hurt her every time he mounted as she was very uncomfortable and spooky at that point.
We have cured her of that but I have another problem:
She walks off comfortably but after a while just stands still, refusing to go forward. It could be that it is all unfamiliar terrain (open country), but no amount persuasion or turning in circles gets her moving. I am reluctant to use a switch and, when I do, she just kicks out.
I cannot see that she is in any pain and is otherwise good natured.
To discern the reason for certain behavior, one must pay careful attention to the circumstances not only at the exact time of the behavior in question but, also, the circumstances leading up to this moment.
First, congratulations on helping this horse become more better for mounting. I hope the horse stands comfortably while waiting rather than simply standing rigidly still. There is a difference, and the difference may be an indication of the reason for the behavior in question.
When you say, the horse "walks off comfortably", do you mean she begins walking in a relaxed manner -- regardless of speed -- or simply walks off readily? Again, there is a difference.
Pay attention to what happens "before" she stands still and refuses to move forward. Does she first slow her pace? Does she look in a certain direction? Does her breathing change? Do her ears point in a certain direction? Do her muscles become more tense?
All of the above signals may indicate growing anxiety over the environment.
Pointing her ears backwards or looking back at you or her own body may be signs of discomfort. These are not the only signs, however. These signs may also have difference causes. Many subtle indications must be viewed together for successful diagnosis.