You see, lately this little mare has been fussing with the bit quite a lot. She'll chew on it (not in the soft relaxed way), shake her head, and pull against my hands. She seems to do it more when I'm asking her to go a little slower and think about something like bend, moving her haunches or shoulders independently, etc. These are things she needs work on and doesn't understand fully yet, but she seems to get frustrated and start fussing with the bit when I'm asking her to think about some of these things. I ride with a light hand, though when she pulls on me I do not yeild.
I'm not sure how lenient I should be. It's hard for me to tell if she's just giving me attitude or if she's really starting to get upset or overwhelmed. I could "kick her up" or tap her with the whip when she's pulling on me and insist that she work through it and do what I'm asking. Or, I can go back to easier work. Today, I asked her to trot out on a longer rein when she was starting to get particularly fussy. She'd keep fighting for a little while, but after a lap or two would settle out once she realized I wasn't asking her to do anything more than trot out without running away. Once she stopped fussing, I resumed the "harder" work until she became fussy again (which didn't take long at all).
I've been using an eggbutt french link snaffle, though have also used a single joint D-ring snaffle and encountered the same challenges. I've fiddled with the adjustment in terms of how high or low the bit sits in her mouth. I don't know how possible it is that she might have dental issues, as she does not belong to me and I don't know any of her history.