If you are unable to receive email notification triggered by the plugin then there could be several reasons for this. We are going to list the most common issue here.
Emails going to Spam Folder
Sometime your email might move to the spam folder. While there are many reasons why an email is marked as spam and would be a different topic to discuss. You might be looking at your Inbox while your email might have been received on your spam box. So make sure whether your emails triggered by the plugin are on spam box.
WordPress Installation unable to send Email
If you have recently setup your site then there is a high chance that your WordPress installation is not set up correctly to send the email. This may be the reason, your contact form or registration form plugin might not be triggering any email. To test this you can simply install this very plugin called Check Email
Once the plugin has been installed and activated.
Go to Tools->Check Mail
Under Send Test Email To enter an email address where you would like to receive a test email from your site.
Then click on the Send Test Email button.
If your WordPress site SMTP is correctly set up then you would receive a test email. If this fails then you need to contact your host regarding this.