What lies within

I'm finding plates to be quite challenging. Who'da thunk? This cross-section photo shows some of my trouble. There's a hump in the middle! It's a strange phenomenon, but each time I throw a plate, somehow the thickness dips down as I near the edge-- but before I've actually formed the edge, so there's still a good bead of clay to form into an edge. That crown is mysterious, and so far I've yet to find out how to move it outward. I'm using a wooden rib to flatten the upper surface and I can feel the downhill slope as I move toward the edge, but can't seem to convince the main body of the clay to move outward, only some of the upper surface is mobile.
I sent a bunch of new pieces to bisque yesterday. I've had better luck with smaller plates (up to about 8"), but these large ones have a steeper learning curve.