My posts used to allows comments, but now there's no comment box. The only change I made was to the permalink structure, from the default to /%year%/%monthnum%/%postname%.html. Also, permalinks aren't working properly; the link takes me only to the top of the page, not to the individual post. I wanted the pages to be HTML pages and used the .html based on information on a Web site abut using permalinks in WordPress.
Comments are on in the Administrative area.