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You need to confirm that the URLs are in fact 100% of your URLs going into the site map are accessible. if it's a big issue in a big site send me the URL in a private message I will use deep crawl to create a XML sitemap for you. The screaming frog tool is excellent as well though does performance well with extremely large sites. check your robots.txt file this so great tool if in case you have more than one (it happens) http://www.internetmarketingninjas.com/seo-tools/robots-txt-generator/ or http://tools.seochat.com/tools/robots-txt-validator/ so many great free tools are found right here http://tools.seochat.com/tools/ It could be a number of things although it could be Google being finicky. Run the site through Moz crawler, use feedthebot.com using "tools SEO" or download the free version of http://www.screamingfrog.co.uk/seo-spider/  this will tell you if there is an issue. If your site is static you can even create an alternate site map with screaming frog if your site is large use deep crawl or Moz analytics be certain there are no sitemaps redirecting to each other so no redirects going from the old site map to the new site map. Make certain that the site map is in an XML format e.g.  http://example.com/sitemap.xml or if in a different format like https://example.com/sitemap_index.xml make sure the proper format That resolves when you look at the site map is what is going into Webmaster tools. Be certain the site map does not contain over 500 URLs per the site map so example.com/sitemap1.xml and so on keep  numbering them appropriately. sometimes Google is overloaded and does not seem to like to play well with certain site maps or the site map is maybe not generating very well on the server and that is fixed later on. If this is a long-term problem speak to your host or developer. My recommendation is if you've done everything I have talked about that you attempt to submit is the sitemap to to Webmaster tools or  simply build a new sitemap and submit that. so if worse comes to worse take the screaming frog and use this URL to send it to Google http://www.google.com/submityourcontent/business-owner/ I hope that helps, Thomas