After long hours of trial-and-error, I finally found out why this didn't work on the site I'm currently developing: the plugin uses the tag DISPLAY name instead of the actual tag CODE.
For example, I have a "intl-news" tag that's displayed as "International news" on the front end. The plugin I should be using the "intl-news" code, not the display name; that will keep everything from falling apart if a user ever decides to rename a tag.