The pagination is missing from tag archives and search results, but it's present in the category archives. The theme uses the same template file for categories and archives, so I'm puzzled. I've double-checked, and all template files call the wp_pagenavi function, which comes from Lester Chan's WP-PageNavi plugin.
In my case, the theme has added WP-PageNavi's functionality to the admin-setup.php file. I have not installed the plugin separately. (In fact, I get an error when I try to activate WP-PageNavi b/c the wp_pagenavi function has already been declared.)
Any suggestions regarding what I should check next?