Sometimes the site tag data used in Share's document library can get out of sync with the tags on the content itself, especially in older versions of the 3.x product.
I came across this today on the Partner Sales Enablement site that we've just launched fully today on partners.alfresco.com, but fortunately the tag scope data is easy to rebuild.
var refresh = actions.create('refresh-tagscope');
Once you've created this file (I called mine refresh-tagscope.js) in Data Dictionary > Scripts, use Alfresco Explorer to navigate into the documentLibrary space within the site you want to refresh, and click View Details from the More Actions menu.
Once that's done you should be able to navigate back into the site document library in Share and you should find that the tag data has been fully refreshed!