This seems annoying or irrelevant thread for the site owners actually; this is not perfect to think the submitting of an XML sitemap on Google will be Indexing confidently. I went through Google Webmaster Help; Google's Gary Illyes says in response to this thread, why a site that submits thousands of pages even though few pages among them indexed in Google.
According Gray form Google help desk,
Your Sitemap must not include the URLs; it can be difficult to find the URLs for the Google Spider, hence may be indexing slowly, but this way to submitting an XML file of your website is recommended.
At the same time you need to consider is that; some of Google Algorithms do not allow indexing certain URLs. If the information appears shallow, may not be indexed totally.
