In 99.9999% of cases it is not the system that has failed, it is an error somewhere else. A lot of the time the user has some rule that is filtering the email into a folder like spam or somewhere else. That has been my experience anyway and the first things I ask are about their email filters and have them check every folder they have.
I have only had notifications totally fail a handful of times. When they did I had to pretty much recreate the notification from scratch and they worked again.
I have had users in my system that got blacklisted/quarantined by Quickbase for some reason and they stopped receiving notifications completely until we reached out to support who confirmed and fixed the issue. I don't remember the exact cause of this though. This is super rare and I have only seen it once. Maybe someone else can explain it.