Total newbie here, but I may be able to give you my experiences which may in turn help?
First I've heard that rock is very hard. I've only now tried Keokuk and some heat treated Texas flint, but you may want to try something different. The Keokuk was nice, but i really like the Texas rock.
I've been thinking a lot about the motions you do when knapping. Wondering what was working and what wasn't and why. First off, the strike is sort of like hitting a pool ball to get it to move sideways. Kind of a glancing blow, with the force going almost perpendicular. That's not the Hertizian cone (spelling?) but the feel of the strike.
I made a huge leap when i started using more of my wrist, rotating downwards as my arm traveled down. Instead of locking it and having my arm travel downwards. It made for a snappier, probably more forceful hit, BUT, without TOO much power caused by my whole forearm. So, almost like a whipping motion. Also, I had been putting the rock at too much of an angle. So when I kept it down, more than my instinct, added with the whipping motion . . all of the sudden i started getting consistent flakes, and MUCH more control. I was getting sweet flakes before, but now with way more control. This then, reduced my hinging and step fractures, caused by too much of an angle so the fracture was digging too deep - and the fracture lost its force.
I'm not at the point of making flake patterns, just getting some flat bifaces. But, I am getting super nice huge flakes. I hope this makes sense and/or helps. If not, ignore me. LOL.
PS-ops, i just saw this again and saw that you were talking about pressure flaking . . . .no help from me then, I'm in the same boat.