WpSubscribers is using JQuery to display popup, but sometimes, the wordpress version, or only 1 single plugin can load an old version of JQuery and prevent WP subscribers to work correctly
for example a customer had a version of FLowPlayer for Wordpress, completely outdated
the jquery version was 1.2.6 but you just need to replace ( most of the time ) the link to CDN hosted of Jquery
here is the list of latest version available
http://docs.jquery.com/Downloading_jQue ... ted_jQuery