Visits from HTTPS websites to my HTTP website are missing a referrer
Missing referrers for the HTTP website
Piwik PRO is able to track both secure and non-secure websites, however, it is always safer and privacy-friendly to have an SSL certificate attached to your website. But if, your website is not secure (Its URL starts with http:// instead of https://) you may experience a peculiar problem with referrers tracking.
Namely, you may not see referrers when your visitors will be coming from a secure (HTTPS) website.
This happens due to the Referrer policy. The default policy prevents sending the referrer data to less secure locations (HTTPS -> HTTP) this means that unless you can alter the policy on the referring website, your web server will not receive the referrer data and will be unable to pass that data to Piwik PRO.
The only way to be able to track referrers in such case is to upgrade your website security by attaching an SSL certificate to it (going from HTTP to HTTPS).