Possible causes:
1. Cookie refused — the visitor had cookies disabled or visited in incognito and wiped the session before signup.
2. Different browser — clicked your link in browser A but signed up in browser B.
3. Cookie expired — the cookie lasts 30 days; signup after 30 days no longer attributes.
4. Used another referral link at the same time — if someone clicks on multiple refs, last-click counts.
5. Self-referral detection — if the agency had the same IP/email as you, the referral was blocked.
Does it seem right to you? Send the signup date and the agency email to support — we can check it in the logs.