Google Webmaster Tools says sitemap is not in a supported format

I installed the plugin and it appears to be working fine. I checked the xml file and it's there and formatted correctly, then I submitted the sitemap to Google Webmaster Tools and it was sucessfully submitted and giving me the Green Checkmark, however when i publish a new post on a particular blog, the checkmark in Webmaster Tools changes to and "X" and it gives the following error:

Unsupported file format
Your Sitemap does not appear to be in a supported format. Please ensure it meets our Sitemap guidelines and resubmit.

Despite this error the sitemap still looks fine when I view the sitemap.xml. Also all the other blogs remain recognized until I publish or update a post then webmaster tools gives an error for them as well. What is going on?