Seconding this. I have the big-picture plot and even scenes of the comic outlined, but paneling them in a way that makes the story flow easily, with pauses and emphasis where important, while keeping the panel layouts varied (so I haven't drawn the same character's face in profile 3 times in the last 2 pages) and making sure speech bubble placement makes sense... that's hard.
(part of the struggle for me is that I watch a lot more animation/movies than I read comics, so I tend to panel things as if they would be appearing in animated format, and sometimes that doesn't translate exactly to a comic.)