You haven't provided enough detail about your setup.
Relying entirely on the the Referrer URL for tracking isn't possible, because sources can suppress part or all of the Referrer for security reasons and your form will never see the data, making it look like Direct traffic. You have to source-tag your links using query parameters.
Please provide more information and sample URLs or pages.
Check your Form settings and URL structure. Also make sure that field isn't blocked somehow.