Both the Event Log and the ULS log are invaluable sources of information when monitoring or debugging your SharePoint solution. Sometimes logging doesn't work. In my experience these are the most frequent reasons:

1. The log disk is full.
2. The application pool account of your web application is not a member of the  "Performance Log Users" group on ALL servers in the farms.
3. For being able to create custom event logs from an event receiver, the timer service account needs write permissions to the following key in registry:
   
   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servicese\Eventlog

If you are just starting to implement logging in your application, a few tips may help you on the way:

• A good logging experience requires some effort, so you may find that using the logging classes in Patterns & Practices SharePoint Guidance can save you some work.
• ULS Viewer is of great help when reading the ULS log. Download it from MSDN: http://archive.msdn.microsoft.com/ULSViewer