Troubleshooting info of the event (Go to https://www.google.com/calendar/render?tab=mc&eventdeb=1, click on the affected event, click More Actions > Troubleshooting Info, copy the text from the pop up window):
What type of notification was not received (invitation, reminder, accept/decline response)?
If reminder, then what method of reminder (email, popup, SMS)?
If invitation, did other attendees receive the invitation successfully? (If so, provide email message headers from invitation that was received.
Is this issue a oneoff, or is the user consistently not receiving invitations/reminders?
Is the user using the web interface or a desktop client (e.g. Outlook)?
Please provide a screenshot of the user's Calendar notification settings.