Older Han/TB gelding....18ish? Supposedly was in the Maclay as a younger guy. My school ended up with this guy which is GREAT for us. The only problem is because he is older, he is great for beginners, so when I get on him (as a novice/intermediate) maybe 1 other girl has worked on putting him into a frame. It seems like this is the thing I struggle with the most....one day its there and another it isnt. Today I rode with nubs for spurs to get him off my leg. We are both "old" so I make sure that we are well warmed up at the trot before trying to get him to frame up. I ask by slightly shortening my inside rein and squeeze and/or bringing it back a fraction of an inch at the same time. My outside rein is held steady with some give and take to it. For whatever reason, if we go long and low, I can alternate squeezing pressure from left to right hand with a slight pull back and he will go onto the bit right away, but when trying to push him into the bit with leg, it seems so much harder.
Other then getting his hocks injected, he is in perfect health. He doesnt drag as much and moves better when we do this. Age was mentioned in the OP...Bart is 18. He isn't over or underweight, his coat has a good sheen, his teeth are floated when needed, his hooves are always done on time (2 front shoes and bare in the back) and he is happy to do his job. He even got wide eyed yesterday because there was fabric fluttering in the wind and he made it look like he was going to have a HUGE spook.
Are you giving him a "stretch" phase after your flexions?? What should be happening is that you flex him (with the wrist only) and the other hand should be willing to give quite a lot. When he "gives", it will be a slight feeling of relaxation on the bit, they you need to feel like your reins are broomsticks and you are pushing the contact out, the neck longer and applying leg to activate the hind. If he is good with it, a slight tap on top of the croup with a dressage whip if the leg aid doesn't work should motivate him. Keep flexing both ways L&L (ie. Inside and outside flexions, both ways) until he is more supple. Focus on creating tempo changes, doing interesting lines and really stretching the back and activating the hind in the warm up, in all three gaits.
Once he is really stretching to the contact, your warm up is over. Give him a walk break and come into the work. Remember that even in the higher frame, the length of the spine should not change, you should still be giving after every rein aid (think of the broomstick reins) and that you should only have one active flexing rein at a time, while the other follows and allows the flexion. In between the flexions, keep your fingers active in the conversation and still giving if he chooses to take contact. The softer you are up front, the more willing he will be to move forward because he will feel less restricted.
Try not to think not of getting him "on the Bit", but of putting your hands within reach of the horse's mouth. The horse will automatically seek the contact.
However, the horse will only attempt to make contact, if your calf aids ask for the hind legs to step up and the seat aids channel the movement forward towards the bit. In other words, you -the rider - have to make your hands available for your horse, that's all. You cannot do this with the reins on the buckle - there must be a connection.If the reins are on the buckle, there is no way the horse could ever reach them, unless he stretches all the way forward and down. This stretching is valuable and desirable, of course, but not all the time. There is a time for stretching forward-downward and there is a time for arching and elevating the neck out of the withers. The difference is communicated to the horse by your seat and legs. Think of the bit as being the forward-most point of your seat - but your seat must be stable, balanced, and connected for this to be able to happen. This means having the elbows connected to the seat too. They must hang off vertical upper arms and rest lightly at your sides. The whole arm (shoulder, elbow, wrist and finger joints) must 'breathe’ with the horse's movement. Otherwise the bit is just a separate piece of equipment whose influence is limited to the mouth alone. But the bit’s action is not just limited to the horse’s mouth. If your seat is connected everything done with the bit travels through the horse’s whole body. You can use the reins to monitor imbalances. For instance; you can learn to feel blockages in the poll, jaw, neck, shoulder, back and even hind legs. You should, literally be able to feel your horse’s hind hooves in your hands and this pattern continually recycles itself from hoof to hand via the seat, legs back, etc. If there are muscle tensions somewhere along the line this recycling of energy will become broken (it will short out) and you won’t feel the whole horse working ‘through’. Durchlässigkeit (throughness) is only possible on a horse whose back is moving in a supple swinging manner. If you can’t feel the impulse of the hind leg in his hand, it is because there is tension somewhere in the horse's body. The horse has to step freely into the bit with his hind legs. When you get this feeling in your hand, you know you are on the way to achieving an elastic contact and having your horse "on the bit".