Try disabling and re-enabling the alert rule. I have one that gets stuck once in awhile and that is what fixes mine.
When the alert is triggered, it should show in the console and you can click it to go to the details of the alert. From there you should see the actions and see if there are any errors. Otherwise, you could dig through logs.
I had stopped getting down/up emails and upon checking, it turned out the email address I had configured for the node somehow wasn't encoded ANSI.