Hello, It started working after I used non www version of my website than www in Google webmasters tools, htaccess and wherever possible.
Now it is accepting the sitemap.xml and accessing all my pages.
Edit 28th december:
Again same problem occurs in the website. Not able to fetch or add sitemap.xml
Note: If I add a sitemap.xml generated using third party tool, then the sitemap is accepted, but not able to fetch any other posts/pages. I tried to add sitemap.php (from an older q2a plugin), which generates sitemap but it also not detected by Google Webmasters Tools.