OnTriggerEnter only called on one gameObject

I'm not sure if this is expected or not. But I have a collider on two gameObjects, I have a rigidBody on one and IsTrigger set to true on the other. What I was expected was for OnTriggerEnter to be called on both gameObjects but it's only being called on one. I even added OnCollissionEnter as a test and same result. One gameObject just receives no events at all.

Is this expected? Or have I just set something up incorrectly. I'd prefer for each gameObject to be responsible for it's own work.

 

I should mention that the gameObject that's not receiving any events is buried pretty deep inside of a prefab. I'm not sure if that matters. Stated just for completeness.

 

I just realized I can post pics. This should help explain my set up.

 

I had to add a rigidbody to both colliders which seemed odd. I read that it's only needed on one but either way, it's resolved.