That is not always the case for the failed alerts. There are threads that I actively read and post in and I do not get alerts on a regular basis. Some are hit and miss on whether or not I get an alert and others just fail miserably.When you stop viewing new posts in a thread that you are watching / leave it unread after getting an alert for it, you'll stop getting alerts, because the system will assume that you're no longer interested in getting alerts if you've ignored one, so you have to go back and read it or mark it as read to start getting alerts for it again.
The only reliable method that I have found is to click on the watched threads link.