I haven't read all the replies yet, but wanted to write this out first then go back and read the thread.
I have a friesian/paint cross, and I do feel that they are extremely difficult to train to go over their backs. I've trained a couple of horses through Second and one to Third, but I've NEVER had this much difficulty unlocking the back. It was no problem on the warmblood or Arabs, the long and low worked wonders. But the long/low work just throws a friesian right on their forehand and at that point you're toast. It doesn't work very well for friesians (imo).
I finally found an instructor that was able to give me the tools to fix my horse. The main points that I took away from the lessons were that the contact HAD to be consistent. My horse will suck her neck into her chest when she gets tense, (not her nose, her whole neck becomes an inch long). The instructor had me change how I dealt with that. Instead of constantly shortening and lengthening my reins to adjust to her neck, I was to simply bring my elbows back as far as I needed to in order to keep the contact the same. She had me widen my hands (not lower, just wider) when the horse was inconsistent with the contact in order to help channel the energy up to the bridle. I had to really drive her forward. I mean BIG FORWARD. Once the horse was consistent with the contact (took about 15 minutes of convincing her to I meant really forward) then I could return my hands to together over her withers. Let me say, that was the very first time I ever experienced true elastic contact with any horse. It was simply amazing.
The second thing the instructor had me focus on was straightening the horse. My mare leans heavy on her right shoulder. The instructor explained that what you need to do at that point is to make them carry their weight on their other shoulder. So we did lots of leg yields toward the left to move her weight over to her left shoulder. The main point being that the horse really
had to move forward
and sideways, with lots of energy. Not the slow pokey leg yields most of us amateurs do. Those are okay when the horse is first learning what you want, but once they have the idea down, then it needs to be used as a gymnastic exercise and not just a trick.
Here's a video of the second lesson with the instructor that ended up helping me with getting my horse over her back:
The sound is terrible. Sorry about that.
And here's a clip from before. This is 1-2 weeks before the video above:
I think you can see the difference in how the horse moves, and how much better she used her back and body in the first video then the second. That was even with me being so nervous at the clinic that I nearly puked (lots of big name riders, I was very intimidated).
It's been almost three full months, and the horse is going like gang-busters now. Totally different horse.
Anyway, hope something in there helped. :)
ETA: Here's a clip of where we're at now: