Another common reason I run into all the time is the link is set to trigger other javascript, which may cut off the DTM code. There are a couple of ways around that, but involve custom code. When I run into the case where other javascript is stopping the DTM Event code from running, I normally see the events Condition statement running (and passing) but then the other javascript interferes before the Analytics/Custom Code section of the rule can run. An easy way to see if this is what's happening is to put a _satellite.notify() message into the events Conditions. If you see that message fire in the Console, you at least know your event rule is being recognized and at least starting to run.