I appreciate this is old but there is a consideration that is easily overlooked.
If you are dynamically creating your sitemap (say php for example). A site can get stuck on 1 url indexed from the sitemap if you don't format the url correctly.
Simple enough, right?
now lets say our variable is:
$SITE = 'https://www.icalculator.info/';
All good right? Wrong... when the php is parsed the result is a url like this:
Note the double // after the domain. This is one of those very silly but incredibly easy mistakes to make. I do occasional SEO reviews for friends / odd contract. the amount of times I have seen that mistake. Anyway, shared now so hopefully if you have the dreaded one page from your sitemap showing in Google Webmaster tools, you can check this.