There are different reasons why the new admin is not receiving the invitation email.
They are listed below:
1. Check or verify in the email logging when you added the new admin in the app if the email was successfully sent out. In order to check if the email was sent out successfully, you can find the information in the Logging Menu as shown in the picture below.
2. If the email is not showing in the email logging, please verify if the email address was entered correctly. once you've verified that the email address was entered incorrectly please rectify it and resend the email.
3. When looking in the email logging, you can search the user by their email address.
4. Once you've entered the email address and you are able to see the invitation email, please ask the user to check their spam folder. 
In the case that the user sees the email in the spam folder, ask the user to make sure to mark our email address ([email protected]) as safe and it should not occur anymore in the future.
If you still have questions after reading this article, don't hesitate to contact us by submitting a ticket.
We're always happy to help ! :)