Comprahensive Study on SiteMap Errors!

Default sitemap file that Googlebot crawl at yourdomain.tld/sitemap.xml
But if you go with WordPress SEO, sitemap path will be at youdomain.tld/sitemap_index.xml

Google XML sitemaps plugin create sitemap with default sitemap.xml
That’s why most of the time this works for all installations. But in WordPress SEO you need to make some changes in the plugin setup. WordPress SEO XML sitemap gives you more flexible and customized options for sitemaps than Google XML sitemaps plugin.

Now have look at your issue:

Step1:
Create robots.txt file in your wordpress directory (if not yet created)

step2:
write the following lines including sitemap url

User-agent: *
Disallow: /wp-admin/
Disallow: /wp-includes/
Disallow: /wp-content/uploads/
Disallow: /wp-content/plugins/
Disallow: /wp-content/themes/
Disallow: /wp-content/cache/

Sitemap: http://yourdomain.tld/sitemap_index.xml

Step3:
In your site admin’s plugin dash board goto WordPress SEO plugin options
SEO>>Permalinks>>Redirect ugly URL’s to clean permalinks.
Un-check the above option and save the settings

Note: clear chache is a must now

Now check your sitemaps at
http://yourdomain.tld/sitemap_index.xml
If it works!? then check the following links too..
http://yourdomain.tld/post-sitemap.xml
http://yourdomain.tld/category-sitemap.xml
and others like for pages, feads, authors etc.
If all is well, then go to step 5

Step4:
If step3 don’t give you the result u are expecting, then goto W3 Total Chache (if installed) dashboard and go for
Browser Chache>>Prevent caching of objects after settings change>>
Un-check the option

Note: clear chache is a must now

Now check your sitemaps at
http://yourdomain.tld/sitemap_index.xml
If it works!? then check the following links too..
http://yourdomain.tld/post-sitemap.xml
http://yourdomain.tld/category-sitemap.xml
and others like for pages, feads, authors etc.
If all is well, then go to step 5

Step5:
If all the sitemaps are verified then goto webmaster tools (by google, bing etc.)
Submit your sitemap URI as
http://yourdomain.tld/sitemap_index.xml
Then fetch as google (let the Googlebot verify)
Now you successfully verified. Here after no need to worry about your sitemaps. Any change in your posts or pages or feeds will be crawled by bots.
Note1: Step5 is a request for immediate indexing your sitemaps. But not necessary, since we have already directed the goolebot and other bots through robots.txt
Note2: Not to forget to restrict the bad bots accessing your sitemap with .htaccess rewrite rules.