The mare I've been working with lately is horrible about holding her canter lead in back. She never strikes off disunited under saddle, and the issue gets better with fitness.
When she was fully fit, my old trainer could get her to hold her lead with little issue. That said, she's been sitting for a while.
If anyone could put their two cents in on exercises for fitness or if she should get checked out (vet, chiro, massage therapist), that'd be the most awesome thing in the world right now.
My Arab was terrible at disuniting in canter when he was younger, it really is all about strength issues, as the horse gets stronger she'll hold it better.
The best thing you can do for her though is make sure she is 'straight' when cantering circles ie don't let her pop out or drop a shoulder. This is when she's most likely to disunite.
My guy now very very rarely disunites (only if something goes really wrong)
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Get her hocks/stifles checked out. It is usually one of those problems when they fall off the lead in the hind end.
I would have a massage therapist come outand do an eval or even a session. Ive seen them work wonders especially with this type of issue where it cpuld most definitly be a muscular issue or even musco-skeletal which they can totoally help with.