Hollow body rocks

I am having a hard time getting my lower back to round for any of the hollow body rocks, bent or straddle.  I don't get a nice smooth role like the individuals in the videos.  It's more of a flat flop, regardless of how intense I contract my abs.  I've been on the program since April of last year and have been very diligent with it.  

You should post a video. I'm assuming you don't have enough ROM going on, so you contract maximally in a position where your back isn't round enough. If you've truly mastered PE1, all PE2 differs is the movement aspect. There's a chance you've lifted your shoulders off the ground, but not the shoulder blades. Both need to be off the ground to have proper form. Your tailbone and spine is all that touches