There are different reasons why this could be happening. You can find them below:
1. The email template was not set as "active": In order to check that, you have to access the email template (via settings -> email templates and then choose the one you are checking out) and check whether the "active" checkbox is checked or not. If not, you already know what the issue was.
2. Check the filters of the email template: In case you are using filtered emails, make sure that all the necessary filter items are assigned to each filter category within the email template. Also make sure that those items are also assigned to the group of onboardees that you expect to be receiving the email template:
3. Check the timing: In the case we are talking about a "new content notification" email template, make sure that the content in the app has the right timing assigned. If for example, the timing for a content is set as "always" no email about it will be sent.