There are several reasons why this could have happened:
1. Your friend did not use your Unique Affiliate Link during registration.
2. Your friend used your Unique Affiliate Link but changed the computer he/she used to sign up.
3. Your friend used your Unique Affiliate Link but deleted his/her cookies on the computer.
...or, which happens more often than not...
4. Maybe your friend has been counted towards a pending successful referral, but hasn't reached the 5 post minimum requirement to be counted as a successful referral.