Objects not colliding twice

Hey Thanks for taking the time to look. The problem I'm having is that I have an object that runs at the player and when it hits the player it backs up and then runs again. The first time he smacks into the player it works fine and he starts backing up. But when he runs again OnCollisionEnter()doesn't get triggered.

So basically OnCollisionEnter() only gets called the first time around and not the second even though it's a clear hit.