As for where the tabs come from, you could (re)read my answer to your similar question from a few month ago, but that was geared at a programming perspective, while this sounds more like an administrative problem.
The most common reason for a tab not showing up is that the current user does not have the right to use the functionality the tab offers - you should check your permission settings, especially if the site in question uses one or more specific access modules.
Another possibility is that the function the tab offers is not enabled for the content type in question, e.g. if 'use revisions' is not enabled for a content type, the tab won't show up (and IIRC, it only shows if there are already revisions available for that node, so if a node has not been edited yet, no tab is displayed)
A third place to check would be the page.tpl.php file(s). The tabs are available there as the $tabs variable and usually printed somewhere at the top of the content region. If your site uses multiple page template files for different situations, it might be that one of those simply doesn't print the $tabs variable.
A forth possibility would be a xyz_preprocess_page() function within a custom module or theme that explicitly clears/overwrites the $tabs variable for some special circumstances.