The Invite-A-Friend feature can be very sensitive since it 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 Stepler-user
• Make sure that your friend goes through the referral process in one go as to not interrupt the process
• Ask you friend to click the link you sent again

If the steps above do not work, please get in touch with us at feedback@stepler.io so that we may investigate the issue further.