If your website uses the HTTP protocol, also known as non-secure, you might get incomplete referrer data. This happens due to the referrer policy, which blocks the transfer of referrer data from a secure (HTTPS) website to an insecure (HTTP) one. Consequently, your web server won’t receive this information, making it impossible to pass it on to Piwik PRO.

If you own a website with HTTPS, you can adjust the policy to allow data transmission to non-secure websites, although it’s not the recommended route. It’s wiser to secure all your websites, eliminating the issue of missing referrers.