• They didn't open your link before signing up. The link to you is made at sign-up and can't be added later.
• The cookie expired or was never saved. It lasts 45 days and is only saved if the visitor accepts marketing cookies (a sign-up in the same visit still counts for you).
• Another link came first. The first affiliate link they opened wins.
• They haven't paid yet, or the payment was refunded.
• It's your own account or one of your addresses: that never counts.

The full guide: why a referral is missing