Yeah, there are many factors that go into accepting the bit. Have his teeth checked, try different types of bits - some horses like single jointed, some like double jointed, some like mullen mouths. Some are fine with steel, some prefer rubber or plastic.

Of course, teaching a horse to be more responsive to the bit has more to do with his body than the bit. If he's stiffer on one side than the other, he will brace on that side and be less responsive. Getting him to soften at the jaw and poll has more to do with getting him to use his hind end and relax his back than with the bit.

My daughter has a 4 year old pony. He tends to go with his nose a little high and poked out, so I started riding him, teaching him to relax his jaw, poll and back. I started with simple lateral work - teaching him to bend in both directions, go straight, then turns on the forehand and simple leg yields. I also taught him to lengthen and shorten his stride. All of that takes a lot of leg, applied correctly. He's now very responsive, does canter departs from the walk, does downward transitions from the seat and leg, and carries himself much better, which is the key to having a soft mouth.

However, my daughter can't get him to do those things because she doesn't know how to use her leg correctly and efficiently yet.

I remember the first time I saw a rattler curled up in my path. This one didn't look like a rattler, but I was still thinkin' 'snake'.