Did you refer any other players who have the same IP with the player in question, or in other words anyone who access internet from the same place? It's possible to get only one referral reward per IP.

I haven't referred anyone else so no, they're the only one; that and they don't have the same IP address as me (I'm in a suburb and they're about an hour away in the city) so there's no way they could have been the same 

Ok, thank you for the information.

I checked this user, and they are registered without referral link.

Yeah no worries; and how? I gave them the referral link and I saw them use it to sign up (we know each other in real life).

Blarf. It's fine; maybe it just didn't register or something that it was actually used. Oh well. Thanks anyway.

It's not possible for a code not to register when used. It's a very simple process, there's no chance for fail. When registration form is submitted, it checks the url and submits the id from there as well. Even the check for IP doesn't affect this, it just prevents the gold coins to be sent later. It's just a plain parameter in the url, not a cookie or something like this that may occasionally fail depending on the user environment. The only possible way for it not to register if there's no id in the url and the link is plain registration instead of referral. Like if a user don't put this link at all, or uses it but registers in another tab without it, or decides to browse other links and then returns to registration through menu or homepage etc.