If alerts aren't working, there are a number of things you can do to troubleshoot. Here are some suggestions - and feel free to check out our support pages for more information.
View the Alert Events Log
The alert events log ("File" -> "View Alert Events Log") provides a summary of any recent alert activity. This prompt will display the date/time of any alert occurrence, the target the alert fired for, as well as a description of the alert. This can be a great help in troubleshooting alerts, as it allows you to see if/when an alert attempted to fire.
Make sure the alert is tied to the correct target
By far, the most common reason that an alert isn't working is because it isn't tied to the correct IP address. If you suspect this may be the case, select the target you're troubleshooting the alert for, and open the alert panel. Here, you should see any active alerts for the target (and if there are no active alerts, you can add them from the alert library from here).
Set up an alert that will fire instantly, with an event that is very evident.
If you have an alert set up – and tied to a host (see above), but it seems like the alert isn't working, then changing your alert parameters (or create a new "test" alert). Set up "Traces to Examine:" to 10. Alert when "1" or more traces are over 1ms. Unless your network is responding in 1ms or less, this alert will fire on the first collected sample with the alert enabled.
For an event type, use "Play a sound", or "Tray icon change/notification" as both of these events happen immediately with no wait. In addition, for the "Play a sound", use "each time alert conditions are met (repeating)", as this will continuously make sound, rather than just when conditions start / stop. Using this sequence, you should be able to tie an alert to just about any host and have the alert conditions fire immediately. Now, add on another event type (IE: email). You can leave multiple events tied to a single alert – that way you can continue to hear the sound while you're troubleshooting another event type.