Google silently changed the AdSense referral program and added a limitation on what counts as successful referral. Before the change any sign-up to the Adsense program would entitle the referrer to a $100 fee, but now Google changed the rules and now the site that you referred has to earn $100 in 90 days in order for you to get your referral fee.
This limitation is a blow to websites that refer bloggers, since blogs usually don’t make a lot of money at first and it usually takes them more than 90 to earn the first $100.
In the referral page (at the bottom of the page) Google added:
An AdSense referral is counted when a publisher, who has never previously enrolled in AdSense, creates an account and earns at least $100.00 within 90 days of sign-up. The referred publisher must be eligible for payment to qualify as a successful referral.
This change might come in order to help Google fight fraud by users who sign for a bogus account, but 90 days is too short especially for new sites that need time to get noticed and draw users.