The Google sitemap displays a tree structure. As a result, some documents might not be displayed if they are under a document that is not part of the tree structure. For example, if you have cms.news documents under a cms.file document and set the ClassNames of the Google site map control to cms.news, they won't be displayed because their parent document is not included in the displayed content.
We have to keep this functionality for security reasons, but there has been, since hotfix 6.0.9, a new property called HideChildrenForHiddenParent which can be set to false if you want to display all documents specified in the ClassNames property regardless of whether their parent document is displayed or not:
<cms:GoogleSitemap runat="server" ID="googleSitemap" HideChildrenForHiddenParent="false" ...