Not all blog themes display an author name ie. a byline at all. Previously, there were theme specific variations. Also those themes that did display author bylines were changed by Staff not long ago. Regardless of the theme, and provided the theme is designed to display a byline, bylines will only display now if there are at least two authors who both have at least one published post in the blog.
See author byline and profile displayed or not > http://wpbtips.wordpress.com/2009/08/15/author-displayed-or-not/