Try this troubleshooting guide

http://sharepointalert.info/troubleshooting-sharepoint-alerts/

[![SharePoint Alert troubleshooting flowchart][1]][1]  
<sub>(source: [sharepointalert.info](http://sharepointalert.info/wp-content/uploads/2009/11/flowchart.gif))</sub>  


  [1]: https://i.sstatic.net/Qdudt.gif