There are many reasons why referral targeting might not work, which all relate to how referral targeting works on the web. In short, the HTTP referer field could be empty for a number of reasons, including if the website visitor:
entered the site URL directly into the browser address bar, rather than clicking on it somewhere else
visited the site by a browser-maintained bookmark
visited the site as the first page in their web browser
switched from a https URL to a http URL
has security software installed which strips the referrer from all requests
If you have any further questions about why your referral targeting message might not be working, our Customer Success Team are always here to help!