Up until now, Patient Data Imports didn't set a "referral source type", so even if the imported patients had referral sources, those wouldn't show up on any of your reports. The only way to fix this was to edit the patient and then add a type (such as "Patient", "Contact", "Other"...). This could get annoying pretty quickly!
But we've fixed this! Referral source information in imported patients will automatically be assigned a type of "Other", ensuring it shows up in the detail screen and reports later on.