Create an Alert in a Library and the first email is sent letting you know you have setup the Alert.  Change or update an item in the Library and the Alerts are not being sent…

First I looked in Central Administration at Job History (_admin/TimerJobHistory.aspx) and not noticed nothing had ran in a few days.

Next, I located and ran the Immediate Alerts job.  After the job ran, I refreshed the page and noticed the Last run time: had not updated.

Well something isn’t doing it’s job!

Now, pop over to the Server Manager –> Configuration –> Services and locate the SharePoint 2010 Timer service.   If it’s not running, right-click and select start.

Cruise back over to your Job History page in Central Admin. and refresh the page.

The same solution might apply to SharePoint 2007, but I resolved this in SharePoint 2010.

more info:

http://technet.microsoft.com/en-us/library/ee924649.aspx

***UPDATE***
For some reason Alerts were not being sent from one of our Web Applications.
After digging around the Farm, I learned that that Web App in question, was trying to send Alerts from one of the App servers in the Farm. Why? Not sure yet.

The server that the Web App was trying to send from, was not allowed to relay emails from our SMTP server… Once we added the IP to the SMTP server, the Alerts started alerting.

***** Another Update *****

One of my users reported that Alerts were not being sent to an AD Distribution List (DL).  Turned out someone had messed with the site permissions and set the DL to Limited Access.

The DL you are wanting to alert, needs Read access to the SITE and the List / Library you are creating the Alert on.  Yes, this sorta sucks if you have a ton of Lists / Libraries that inherit permissions from parent.