I would just try changing the bit up until she likes it. She just might not like having a single jointed bit, and might prefer a mullen mouth or french-link. Or it is possible she doesnt like how thick/thin the bit is in her mouth and isnt comfortable with that either.
My2Geldings....why do you think a full cheek is not a gentle bit? (just as a question as a sticky in the tack and equipment posts it as a fairly mild mouthpiece still)