Have teeth etc all been checked? Is the horse comfortable in the bit and does it fit well? Is it a recent occurrence? How does she avoid he bit? A grackle tends to stop them crossing their jaw, whereas a flash or drop will just encourage them to keep their mouth closed xx
I would say if there is not a physical issue then there is a confusion issue on her part - there is always a reason. A drop is definitely the best place to start - a grackle is for preventing jaw crossing. The drop steadies the bit in the mouth and, if correcty adjusted, gently stops the horse opening its mouth. The flash was a poor compromise to allow some of the action of a drop but provide a cavesson to attach a standing martingale to. The auction riders of the 80s saw it as a way to present dressage horses more quickly and get them sold and the world followed them like sheep!
Try a softer bit. A grackle or flash will only clamp her mouth shut - why would you do that when what you need is Mobility in the jaw for her to accept the bit.
Find a classical trainer and work with her anatomy, not against it.
Clamping jaw closed means she can't swollow, can't breath properly and can't accept the bit.
Not sure I agree with this, sorry - why would 'clamping' her jaw closed mean she can't breathe? Horses can't breathe through their mouths...
If the flash/grackle/drop is stopping her breathe, then it's fitted incorrectly. In fact, if it's 'clamping' her jaw closed, then it's fitted incorrectly.
IMO, if a horse is opening its mouth to avoid the bit, and everything has been checked to make sure there's not a problem, then a tack change is the obvious next step, but without seeing what she's doing, it would be difficult to suggest what's the best route.
Do read around - there are many variations on a loose ring snaffle - and check that her bit suits her mouth conformation and her stage of training. I will say that the flash and the way it is designed, really does not do a good job and to be effective both parts, but especially the cavesson, MUST be pretty tight. A drop can be much looser and be effective because it is designed for purpose and the angle and action of the two nose parts actually work. Most classical dressage bods really don't like the flash for that reason.
I do agree that this is probably mostly a schooling issue - she expresses her lack of understanding, or protests that what she is being asked to do is too hard, by opening her mouth. A drop will help correct her relationship with the bit, rather than just trying to keep her mouth shut. The Spanish Riding School start all their young horses in a dropped noseband.
My 3yr old is in a grackle - it has solved the problem. She learnt that if she opened her mouth wide enough, she could get away with murder. A loosely fitted grackle has stopped the wide open, and allows her to move her jaw still. Shes now learnt to not bother with the open mouth trick, and is about to go on her holiday.
I have her mother in a drop, to prevent her putting her tounge over the bit - again loosely done up.