The most common cause of this is that email notifications have been disabled for the user. To manually enable email notifications, click on People then on Users. From there, find the user in question and click on the Edit button. Click on the checkbox beside Email Notifications and the user should receive all future notifications.
If they still do not receive any notifications, there are several other reasons for this:
The email that you are trying to reach has been incorrectly entered
The email has been flagged as spam
The email service that the learner is using has been set to only allow incoming emails from specific domains - this is unfortunately completely out of your control and you will need to either have them or their IT department white-list emails that are being sent from your platform