Google is showing a yellow triangle "warnings" in web master tools against the sitemap. I can't see anything wrong - it appears to be downloading it and saying success - last downloaded 15 hours ago. I can't see anything odd in the Google diagnostic panel. This is the site map: [external links are visible to admins only] I tried resubmitting it and waiting a day but it is still showing the yellow triangle. My traffic is going down too!
There must be errors with your sitemap. Check the code in your sitemap to see if they are in the correct format.
Just in case you don't realise - if you click on the name of the sitemap (eg /sitemap.xml) it then tells you what the problems are that Google is reporting. I didn't know this so spent months trying to find out why I was seeing a yellow triangle for different sites - I couldn't find the answer in the forums possibly because other users assume its obvious - things always are once you know!
There is an error with your sitemap or one of the URLs in your sitemap.Maybe there is a URL that takes more time to load than usual. I had the same error on my site and I searched for every possible reason and this was what I came up with.