Ask H&H: bits for showing

Q: I have owned my 15.1hh, Irish Draught-Thoroughbred, eight-year-old mare, whom I compete in novice ridden classes, for six years. She has always had trouble completely accepting the bridle, resulting in back problems. I have tried both the double bridle and the rugby pelham, but she has a big tongue and is only truly happy and obedient in a hackamore. In the show jumping world you can compete in a hackamore, but not showing. Why is this?

CN, Herts