If you are using online services such as http://www.seorobotics.org, http://tools.weburlopener.com, SEOquake, etc to generate SEO stats on your site you will be prompted that the XML sitemap is missing.
This is pretty normal because online SEO services are only capable to detect a static sitemap.xml file located at the server root.
This is a deprecated approach to manage sitemaps, that requires manual updates of the sitemap file everytime a new content is added. Using a modern approach of dynamically generated sitemaps, instead of having a static file to update manually, online services report the sitemap as missing because no static files are present on server.
You need only to ensure that the sitemap entry is present in the robots.txt, search engines use robots.txt to find and fetch the sitemap on your site.