Board Thread:Help desk/@comment-5677091-20121029101228/@comment-9713132-20121029102015

Several possibilities:

There is a simple delay in the system, prohibiting you from redeeming that last referral. Perhaps waiting a few days could give you that final referral.

While there is meant to be no difference whether the referrals are from EU or US servers, it's not explicitly stated (and therefore I'm unsure) if referrals would count from different sub-servers, eg. the EU West and EU North-East. That might be the issue.

There is a glitch in the system where you have been denied your referral, for whatever reason (an invalid referral, or some error in your referrals, for example).

The best response to simply raise it up with Riot on the official forums. Hopefully they should be able to find the problem, and fix it should they consider it necessary. I do suggest though you double-check everything - the referral system is pretty solid and very few bugs have occured in its history.

Hope it helps, mate.