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)
Well, I don't typically feed during training unless it's a small treat during a break or something. I did give her a small treat, a little apple pellet thing, the first time I put it on her. It just distracted her and made her pushy. I don't want her to relate getting the bridle on with being fed - just like any part of the training. That's the main reason I don't feed-n-train. This filly is smart and if I give her a firm rub and a "Gooood girl" she knows she's done something right! :)
I never really thought a full-cheek snaffle was a harsh bit? No shank, no twist, plus it has the bars which keep the ring from sliding in her mouth. I've always used this bit to soften up my OTTBs and it's worked wonderfully, and I've broken out a couple of young horses with it as well.
Someone suggested that I run a leather strap through the rings of the snaffle and over her nose, so that when I pull back I get pressure on the bridge of her nose as well (like she's used to with a halter). Don't know how I feel about this... I am picturing the strap slipping down and pinching off her air supply, and the bars of the full-cheek getting stuck under it... I don't know. But I understand the concept... any thoughts on this?
Generally speaking, you want to start off with very gentle bit. Full cheek it's quite rough of a start for a horse who's just being broke. Get yourself a loose ring snaffle, or a loose ring french link(my preference). They are a much better start I think and you can always change bits and get something a little stronger when you start riding her.
A full cheek is an excellent starter bit - It's a snaffle, so just as mild as any loose ring, plus the full cheeks prevent it from pulling through and aid a young horse in understanding lateral aids.
How thick is the bit? My first thought is that she has a low palate or thick tongue and there might just be too much bit in there, preventing her from swallowing or closing her mouth properly.
I started Latte is your average french link loose ring, as thin as I could find - She went okay but I just wasn't happy with it. I switched to a level one barrel Myler loose ring snaffle - The extra thin-ness and less play were exactly what she wanted and she quieted her mouth a great deal. However, I have been bitting her for about 10 months and riding off the bit and she is only now starting to really soften and not chew. Though I am doing things much slower then most - My point is some horses do take a long time to get used to the idea.
Not very thick. I guess it's average? I'll take a picture of it next time I go out to the barn. Her owner previously bitted her with a grazing bit (medium port). I told her that was a big no-no for such a young'n. Those long shanks on that poor baby :(
I've seen a lot of people put tom-thumbs in as their first choice on fresh babies. I don't know ... I just feel like there's no contact there. Not the right kind of contact, anyway. I want the horse to feel my hands, not the shanks and the curb chain.
*Quick suggestion to Hackamore users!* I replace the curb chain with a leather strap. Horses REALLY appreciate it!
Below is a picture of the filly the first time I bitted her up. (The throatlatch is under the reins so that they won't fall by her feet and get tangled if she puts her head down while I'm longeing her.)