Yeah's he's an ottb, and was rehabilitated 5 years ago,
Green horse jumps 3'6" currently, 8 years old, gets chiro work every few months, on two joint supplements, stomach ulcer supplements, hoof, and coat supplements too. Works 6 days a week with one day off.
No back soreness when you do the touch test, sound all over says the vet.
Bought breast plate thinking the saddle could be sliding back onto tender muscle, but now riding with it, the problem hasn't improved.
Have you thought about this being a balance issue? IMO, that's what it sounds like. Does she seem to do it more to one direction? Do you feel that you are balanced when you ask for the transition? Or could she be weak in either hind that starts the depart? It sounds to me like the head tossing could be a result of any of these things.
I've had my trainer, lessor, and another girl I ride with ride him and they all have encountered him doing the same when they ask for a canter,
I give a little pulse with my inside rein to assist him pick up the correct lead, and keep his attention on me.
Because so many people have encountered the same issue I don't feel it's a "rider problem"
Balance is always an issue with riding in general, so I suppose it is a possibility this is a component, but without much changing in the ways of training, there shouldn't be much reason to have a balance change, and since this started fairly recently, I can't help but think it may be something else?