When a new lead source is "unknown" that means that the person entering the lead form did not enter any information in the "how did you hear about the club" field. Due to this, we are unable to see the exact origin of these leads as this information is not found in our database.
To avoid future cases of leads coming in with no referral type, updates can be made to your Add Lead Widget settings to make the Include 'Heard About Club How' field a mandatory field to be filled out on external sites.
Navigate to Setup > Leads > Add Lead Widget. Select Yes for both Include 'Heard About How How'? and mandatory?.