If things don’t work, you should check for these common faults…
Does the page you’re viewing link to the sitemap? View the source of the page to check for the appropriate
<link> elements if you used that method. Use the developer tools of your browser to check that the page was served with the correct HTTP extension header, if you used that instead.
Is the sitemap file reachable by the visitor?
If you’re using our own Firefox extension, you'll see it has two “Source code” buttons at the base of its sidebar. One displays the sitemap source in the current window (with an XML DOM interpretation), while the other shows it in its own window (with syntax highlighting).
Check that the server is delivering the intended content with the correct MIME type. Check that the visitor has permission to receive the sitemap. Look for XML well-formedness errors (including charset mismatches).
Use the validator to check that XML elements and attributes are used correctly.