lia-mark-empty class

Does anybody know why components get marked with the ‘lia-mark-empty’ class and contain no content?

I wanted to add the ‘primary-post-button´ component to our home page (category) quilt, just like it is in the ForumPage quilt.    Yet when I do this it does not display, and instead puts an empty DIV there with this class name.    Works fine on the ForumPage, but not the CategoryPage.   Why?