I think a nock-high condition that causes your bare shafts to impact somewhat lower on the target than your fletched arrows can be an indication that your string nock needs to be lowered (unless it's already too low, as Mark said), or it can be an indication that your forefinger is pressing down on the nock when you draw the arrow, causing a rebound off the shelf, due to too high elbow or some other form issue.
However, I think extreme nosediving into the ground in front of the target is more of an indication of a tillering problem. I once had a Hoyt Gamemaster that had that problem, and could not be tuned to shoot off the shelf, although the bare shafts flew fine off a rest. I eventually sold it, since I only want to have bows where bare shafts fly well off the shelf. As Tom mentioned, this may not be as important as we make it out to be, but we are only in this for our own enjoyment, and I don't think I could be happy with a bow where bare shafts don't fly well off the shelf, particularly since there are so many bows out there where bare shafts do fly well off the shelf.