Depending on your email settings if someone is subscribed to a Forum rather than a Topic they will either get email notifications for every post in that forum, or they will get an email for the first post in that forum telling that a post has been made in the forum and their maybe more.
If your forum email settings are for the later they will not get another email notification until after they revisit the forum.
You can change the default behaviour of a single notification till they return to the forum in the email settings page in the admin. However, changing the settings so that email notifications are sent regardless of if they visited the forum or not increases the number of emails sent massively and can often lead to being reported as a spammer by past members who can not be bothered to turn off email notifications.