Users won't receive notifications for posts that they make themselves, topics that they've already read all of the posts for, or posts that are within a topic or category that they've unfollowed. If a user has chosen to receive notifications for followed content on a daily or weekly basis in their settings, that notification email won't be sent if it would not contain any topics due to the aforementioned reasons.

If the user isn't able to receive any emails from the forum, that may be due to other reasons.

If the member's settings are correct and the email history shows the notification as sent, the email is being filtered or quarantined at their mail system rather than skipped by your forum. Work through Why isn't a member receiving our forum emails? (debugging guide), which starts from the email history and covers spam folders, email quarantine, and domain authorization. Corporate and university recipients are the most common case. Members can start from I'm not able to receive emails.

Also, the weekly digest email has its own criteria for when it's sent.

To confirm whether a specific notification email was sent, skipped, or failed, check the email history page. It shows each email with the reason it was skipped or could not be delivered.