Recent Global Comments Widget not picking up from Comment Indexer database table

Hi, I'm setting this up for the first time on a test multisite installation.
After sussing out that the RGC widget requires the Comment Indexer plugin (why doesn't it say that on the widget page btw?) I can't seem to get it to show any comments. Here's what I've confirmed:

The widget is showing on the main blog page
the comments are showing on the multisite blog
the Comment Indexer plugin is catching the comments and storing them in the database table.

So everything right up to the widget is working, but the widget just is not showing them. I've disabled / reenabled all plugins and the RGC widget and retested with new comments, and same story - they're in the DB but not being shown by the widget.

The environment is a VPS and I can provide detailed configuration if needed. The test platform is currently 3.01 / Twentyten theme (there's bigtime incompatibilities with some other plugins on 3.1, so I have to wait until those are fixed before I can go to 3.1). I'm installing the widget on the Main blog's sidebar using the default Primary Widget Area.

Any help would be GREATLY appreciated. I have no idea how to troubleshoot a widget - is there a function call in the RGC widget code that I can stick into the template manually to show the comments / error messages or something?