Hi Arnee,
thanks for you reply. Unfortunately the newer posts still are not added to the particular sitemap, like "sitemap-pt-post-2014-03.xml". The "last modified" date is updated, but it's still empty.
I can't remember exactly, but before using your plugin our sitemap was managed through "WordPress SEO by Yoast". I think we switched to your plugin in September 2013. Since then (i guess), most of the new posts are not added to the sitemap and the problem seems to exist even with version 4.0.4. :(
Is there a setting our server does not support or are there any problems known running the Yoast-Plugin parallel (of course i've deactivated the XML-Sitemap function there)?
Kind regards