As long as I've ridden she's had problems with bits, even when I tried her, and her owner rode her, she would flip her head around. She hadn't been ridden english in four or five years until I got her, so I hadn't taken our first summer together slowly but any kind of contact she starts flipping around.
Yes, it could be her teeth. Have you lunged her or ridden her with you on the ground handling the reins, or "in-hand" training? You could watch her reaction. I would try ground training with just a lunging halter and lead and also ground drive.
It might also be that she has a bad habit of disobeying the bit.
My 6yo QH played me by not turning, throwing his head, etc. We had a "discussion" with some lunging for respect (ala CA's method), and I also lunged with a snaffle and reins attached to the saddle and we worked on impulsion on command. After just one session of this, he was listening to my aids and staying on the track in my small arena, and the head tossing stopped. Although I've worked only on ground training, leading, backing and side passing as I cleaned his stall this winter, I will be working him in hand, and lunging with tack as described above before I mount up. THEN, when I ride we will be working on listening to my weight for walk on and for halt for at least one month, but that's bc I want to drill this into ALL of my horses this year.
I would make a list of appropriate leading behaviors, too, and see if there is anything lacking.
Further, whenever in the last 28 years of horse ownership, I have found that my horse prefers a certain bit, I stay with that bc they listen well. $40 isn't much for a really good bit, considering how much the total horse costs.