Unfortunately some emails sent automatically by the site are being blocked by some email systems. I am not sure why this is but am trying to get it remedied.
When this happens I normally get an email reporting it and I resolve the situation manually, but if you do not get registered within a couple of days then email me and I will get it sorted out.
UPDATE: I've been it touch with the ISP that host the site. They are aware of the problem which is related to emails sent from their servers to Microsoft related email accounts such as hotmail and users of Outlook. They are working to resolve the issue.