There can be a number of reasons why this might happen, if you follow the steps below you should be able to identify the issue.
1) Ask the volunteer to leave and rejoin the session. This will resend the notification email to you.
2) If you have not received the notification email in your inbox then check you JUNK mailbox.
3) Ask the volunteer to email you directly to see if you are receiving any emails from them outside the volunteer system. If you dont receive any email then it is probably an issue with your incoming email box, you will need to speak to your IT support to resolve this.
4) As a last resort you should try registering as a volunteer and joining your opportunity to see if you receive an email notification.
If the above steps have not identified the issue then please contact your administrator via HELP > CONTACT US from the upper menu.