This is actually not an issue. There might be several reasons why the referer URL would be blank. It will/may be empty when the enduser:
- entered the site URL in browser address bar itself.
- visited the site by a browser-maintained bookmark.
- visited the site as first page in the window/tab.
- switched from a https URL to a http URL.
- switched from a https URL to a different https URL. (only if it is blocked by
- has security software installed (antivirus/firewall/etc) which strips the referrer from all requests.
- is behind a proxy which strips the referrer from all requests.
- visited the site programmatically (like, curl) without setting the referrer header (searchbots!).