If other people are getting emails, that generally points to something user specific. I assume that these users accounts are in a "valid" state; otherwise, they'd normally get a notice indicating the action they need to take. What does the user's last activity show in the control panel? Is it recent? If not, that might explain the problem.
Beyond that, it may be a mail delivery issue. If there are no mail-related errors in XF's error log, you'd need to look in your server's mail log (if using PHP to mail, something like /var/log/maillog; if using SMTP, somewhere on your SMTP server). If you don't have root, you may not have access to this directly (your host would though).