Actually, it does have to do with where the head is. A horse with his nose in the air is not very able to be on the bit. Nor a horse with his nose cranked back toward his chest. And, "on the bit" does not mean "collection". it just means that the horse meets the bit without resistance or bracing, and without withdrawing from contact with it. His energy should "meet" the bit at the place that the rider has set it. So, yes , the whole body is involved in that.
For the horse to meet the bit, and stay there, he must be able to really trust your hands. So, you have to have really steady hands, which means you have to have a steady seat. those things take time to develop.
it's possible that you are doing a lot of rein "wiggling" to get the hrose to give to the bit, but it is more of you pulling him back, or down, than him going forward to meet the bit, and as soon as he comes back, you release him and he pops up again? think of it more as you put the bit in a good place, then ask him to meet you there. when he does, if you want to reward him, give ONLY the inside rein, just a little , and this can help reinforce to him that he has done the right thing.
never try to pull him back down into a "frame" by pulling downward with your hands. If your horse pops her head up, you follow her up with your hands, maintaining the elbow through wrist to bit straight line alignment, and keep asking with the inside rein for the hrose to soften to the rein, and then, with you leg, ask her to step into that softness and meet the bit, again.