I believe this to be an issue with a jQuery conflict.
Since version 3.2 of WP jQuery has been updates. There might be other plugins you are using that doesn’t play nice with the new jQuery version which is then triggering a JS error, thus stopping the tag/AJAX functionality from working.
Try disabling all your plugins and see if this fixes the issue. If it does enable one by one to target which plugin is causing the issue.
Once you know what plugin is causing the issue then you can either contact the plugin author for a fix and try asking a question here to see if we can fix it.