Firstly, your location's Notification Alerts must be turned on for your preferred notification method (email or SMS) for any visitor notifications to send.
After this, it will depend on the scenario:
You are the host
You may not have your personal notification settings turned on.
Or your visitor may not have selected you as the Host. A Sign In/Out Manager can check this from the Sign In/Out Manager 'Visitor Sign Out' screen. If you do not appear under 'Visiting' for the visitor, they did not select you as their Host.
Or your email or mobile phone number may not be set up correctly in the system. You can check this from your profile by clicking on your name in the top-right corner of the page.
You are supposed to receive an alert when someone else is the host
They may not have added you to their personal notification settings. They will need to make this change themselves.
You are supposed to receive a notification when no host is selected
You may not be set up under the location's Notification Alerts to receive this. An Administrator will need to add you to receive this notification.
You are supposed to receive a custom notification
There may be a custom notification set up through the Triggers feature which you are not receiving. The trigger actions may not be set up correctly. An Administrator will need to make this change.