There are several reasons why this may occur.
The event you are trying to listen to does not exist in your version of Magento
Someone hacked into the main file and accidentally deleted the event you are listening to
Someone overrides the method
Your observer is not configured correctly and Magento does not see it.
Your observer is configured correctly, but the old configuration is cached
The steps that I will take for debugging are
Make sure that the combined global configuration has the configuration of your event. If it does not clear your caches, until it appears
Download the latest source code and split your app/code/core/ and lib/ into the virgin version. Type man diff from the unix prompt to learn about the diff tool if you are not already familiar with it.
Grep (or ack ) is the base code base for the event you are trying to listen to.
Temporarily add the registration code Mage::dispatchEvent to app/Mage.php to ensure that the event you are looking for is actually triggered.
Starting with Mage::dispatchEvent , follow the execution path to the point at which calls are being listened to and see why the code in Magento does not call your method.
The first time you do this, there will be time, and take notes on what happens in the main Magento system code. Thus, the next time you debug a similar problem, it will be much faster (if you really want to be beautiful, you can share what you find here, on your blog or on the Magento wiki)