I think I know what's wrong... or at least one of the things that's wrong, with that last study. I think I should concentrate more on using a small number of values, preferably just two, lit and unlit, to outline the shapes. That would fit a lot better with the line art, for one thing. I started out like that, but somewhere along the way started playing with trying to render things, and it just didn't work out at all.
Also, as usual, some of those lines need toning down.