Similar content types, each with an importer. One imports the other does not.

I have two content types on a dev Drupal 7.7 install with a very similar structure. I wish each to be populated via the manual import of a different CSV file (TAB delimited, actually). This first worked like a charm. The second will not import a thing. I have exported the importers to a file and attached them. The TAB delimited data files are attached, too.