Hi Sam
May be reason was "Secure websites that link to non-secure websites will not send referral data".
http to http – Referral data sent
http to https – Referral data sent
https to https – Referral data sent
https to http – No referral data sent
Reference : https://www.quora.com/Why-cant-Google-Analytics-track-the-referral-traffic-from-HTTPS-sites