WordPress SEO is a great all in one solution for managing your site. It takes care of SEO, Sitemaps, Robots.txt, and Htaccess eliminating the need for 3 separate plugins.
There happens to be a few bugs that prevent sitemaps from working properly. Google Webmaster Tools may be giving you the following error.
Your Sitemap appears to be an HTML page. Please use a supported sitemap format instead.
If Only Some of Your Sitemaps Result in a Not Found or 404 Error…
For example: http://yourdomain.com/page-sitemap.xml results in a not found error.
Fix:
Edit a page/post in admin.
In the WordPress SEO Panel, click “Advanced.”
Change “Include in Sitemap: Auto detect” to “Always Include”
We don’t recommend this for sites with a lot of pages. Hopefully Yoast figures out a solution soon.
If Your Sitemap Doesn’t Show up at All…
Try resetting your permalinks.
Go to Settings > Permalinks.
Just click “Save Changes”
Give it a try.