The published date is in the future (scheduled), or the record is restricted to a group and answers 404 to everyone else, or you tested signed in and the public gets a 404 for another reason: no active revision (the page lookup requires one), an unpublished ancestor (the path is resolved one slug at a time, each under the published rule), the wrong site, or a rewrite or custom URL intercepting the path. Test with curl and read the status code, not the browser you are signed in to.