[Plugin: Popular Posts] How to debug: "Sorry. No data so far."

I use WordPress Popular Posts as a footer widget on several sites. On some, like BuildYourBlog.net, it works. However, when I use it on ConfidentMan.net, all it ever says is “Sorry. No data so far.”. Anyone have any idea how I can kick it into action?

thanks,
Graham

http://wordpress.org/extend/plugins/popular-posts-plugin/