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The Joomla! Update Notification plugin of Admin Tools looks for two things: the user group must be 8 (the default Super User group) and the sendEmail column must be set to 1 (i.e. the user must be allowed to receive system messages). If your Super Admins belong to a different group and/or they are not allowed to receive system messages they won't receive update emails.

Furthermore please remember that in order to receive update emails you need traffic and there is also the cache aspect. Since this is a system plugin, it gets triggered by traffic, at most once every 10800 seconds (yes, that's pretty much 3 hours). Even then there's only a 20% chance that the plugin will fire; this is deliberate, avoiding multiple emails for reasons you probably don't want to read about. And then again, it will only send an email if an update is found. The availability of updates is cached and the cache is renewed every 6 hours. There's a big chance that the first 9 hours since an update is available, even if your site has lots of traffic, that you won't receive a notification email. If you don't have enough traffic this may be as much as a few days.

You're welcome!