The referral link that Tubbly has generated has several checkpoints in the invite process to prevent cheating. These checks can however fail even on a legitimate referral, what you can try to do is the following:
-make sure they are not a current or former Tubbly user,
-make sure that your friend goes through the referral process in one go so as to not interrupt the process,
-ask your friend to click the link you sent again.
If the steps above do not work you can send us an e-mail at firstname.lastname@example.org and we can transfer your tokens manually.