After submitting sitemaps to Google Webmasters Tools you may be prompted for some warnings or errors.
In this case look exactly at details of the warning to understand possible reason.
Often this can be due to some urls that are blocked by your robots.txt. If this is the case, change the robots.txt disallow rules accordingly or exclude that links from sitemap. In some other cases the XML sitemap is not valid due to XML document not well formed. Ensure that you are submitting the right link for XML sitemap generation and not that used for HTML sitemap.