So I have a well-balanced ex-racehorse whom I can get round (that is, using his back and in a frame), but he rarely comes completely into the bit. He tends to be just shy of it, is very fussy (especially if a green rider is on). I've tried a nathe, a full-cheek snaffle, a fat snaffle, etc., and I can't really get him completely accepting the contact, partly because of his attitude. It is subtle to the eye, but I can feel it when I ride.
Any ideas? With the fatter (read: gentler) bits, he simply keeps his mouth open.
His teeth are good and have been routinely floated with very reputable equine dentists who say he has a fine mouth.
I like the nathes, herm sprenger duo's, happy mouths, a soft rubber snaffle or try different mouth pieces single jointed, french link, mullen or a logenge in the middle. Some horses have preferences on the types of mouthpieces they like. My most sensitive types go great in nathes althoug the first few rides they might be a bit fussy they settle in. Also, have a good dentist check the teeth. Young horses can often be mouthy due to caps shedding or other teeth issues. On the really sensitive guys sometimes they can not tolerate a rider with bad hands or inconsistent contact.
I went through that exact thing with my OTTB. Didn't think it possible for a racehorse to have such a butter soft mouth. I didn't work on it when I got him first and had him listening to my body aids really well. I think I have the whole package now that he's starting to accept the bit more.
I started him in a Myler comfort snaffle then thought he'd like to 'play' with a french link roller, but he hated that so we are back to the myler & coming along well. (IMHO)
My sensitive horse goes very nicely in a nathe, but if that doesn't work for your horse...
How about taking one of the myler bits (thin!) and giving it a couple wraps of sealtex? It would be softer than a regular myler, but still thin.
Perhaps a chiro visit would be in order as well. If he's "out" in his poll or jaw, I'd think he would not want to really come up to the bit. I'd also keep green riders OFF of him. The last thing you need is for him to start to trust the hand and then have him grabbed--the more that happens, the more work you're going to have to do to get him to trust the hand next time.
I had the same issue with one of my OTTB's so I rode him for quite a while in a rubber d-ring then a happy mouth. He finally started taking the bit instead of shying away from it. He's graduated from those now, 4 years later and goes in a D-ring copper roller.
As others have suggested, try some different ones and see what he likes. Years ago I did ride a horse with sealtex wrapped on a bit and that horse went well with that.
I've found with some of my fussy horses that the loose rings actually bother them more then help.... the bit keeps sliding, and it makes them uneasy. My guy actually was LESS fussy in this bit then he was in the nice fat loose ring I had him in before.
My homebred goes really well in a nathe - and he's super soft mouthed.
I would start with a french link snaffle if he feels good on a regular jointed snaffle but just *not quite* on it. And I would tend to go thinner rather than thicker based on your description of him.
I had a horse that sounds similar, and I noticed a big difference when I moved from a fat D-ring (regular jointed) snaffle to a sprenger KK snaffle (basically a french-link and a thinner mouthpiece than my fat D-ring). Then I noticed another big difference when I moved from that to a low port myler. That was the first bit he was ever willing to actually lean on and the one I rode him in for many many years.
Why use a bit at all? This website ( www.animalambassadors.com ) has ideas. And I've found that horses that open their mouths are usually getting hit in the roof of the mouth with the joint of the snaffle (you know how sensitive the roof of the mouth is if you've EVER bitten into a potato chip wrong). So a flexible mouth with NO JOINT. I've also NEBVER seen a horse that wouldn't get along very well with a Pelham. They were the next step for a horse/pony that didn't do well in a snaffle. People seem to be avoiding them these days on the mistaken that they're "too severe" that they ONLY work because they hurt so much. When did you ever hear of a horse doing better in a different piece of equipment because it hurt more?
Like the poster right above me ONLY got a bit the horse didn't back off of when she switched to a non-jointed bit. Those snaffles HURT. Only the nicest horses work very well in them.
When we first got ours, he was in a latex wrapped snaffle, but we have him in the Herm Sprenger KK Ultra and it's worked out great. It is an expensive bit (at least to me!), though, so might be good if you could borrow one first to see how it works for you.