Sometimes it just takes relaxation. I've found horses generally find their sweet spot when you are relaxed and just push them gently. My horse found his miraculously one day when we were sitting trot. Gathered it was the slowest most gentle trot ever.. but he was connected. His spine was up, he was reaching underneath, he had his head frame (without me doing anything special) and yeah we moved like a snail.. but that's how we started. And we built up from there. Lunging in the side reins helped him a lot.
But what I'm saying is, the horse needs to figure it out. If you force them into something.. they'll think that's how it should be and they won't seek out any further.. does that make sense?