Your Sitemap Files Cache Order Needs to be Recoded

Your sitemap structure is like this:

sitemapindex.xml
     - post(_page2..).xml
     - page.xml
     - taxonamy_category.xml

Let’s say the user caches the sitemap file. If, a new post is made then we know the sitemap is old. So lets say we flush the cache. But now, if someone accesses sitemapindex.xml then this file will show the old timestamp for post.xml (meaning, it is unaware that a new post was made). So now, if someone accesses post.xml (which now regenerates) this file will now include the new post and of couse a new timestamp. But when you look back at sitemapindex.xml it is still old. One would now need to delete sitemapindex.xml and refresh it before the timestamp for post.xml (seen inside sitemapindex.xml) is updated correctly.

This says to me that your sitemap cache ordering is not correct. You should either be caching post.xml and the other xml files first before caching sitemapindex.xml.

Or, if the post.xml file doesnt exist then look to the dbase to find out what is the latest post date.