Sitemap is not working

Hello All!
I recently made site using wordpress 3.5.1, and for sitemap and other SEO things I am using Yoast WordPress SEO. right now i am facing problem with sitemap. My latest posts are not showing in google and other search engine. whenever I go to webmaster tool and select on sitemap it shows: Error loading stylesheet: An unknown error has occurred (805303f4).
and when I remove www from the url it shows my sitemap, but still google is not indexing my pages and posts. Please help me out.

here is my site URL: http://www.studentsstep.com/

Thanks

http://wordpress.org/extend/plugins/wordpress-seo/