Trigger not registering imported objects

My scene is made up of many objects, some imported, some simple Unity shapes.

I have an IMPORTED cylinder which moves around the scene, 'Is Trigger' is enabled on the cylinder.

The cylinder has a script which does different things depending on the Tag of the object it moves into.

Here's the problem: It doesn't register any of the imported objects, only the Unity shapes (cube, sphere, etc.). Why is this? Can it be fixed or do i have to apply box/sphere colliders to everything?

• All imported models are .FBX

• 'Import Colliders' is activated in the 'Import Settings'

• The colliders of the imported items react fine against other colliders and raycasts...it's JUST when i try to have my trigger react to them.

I'll put an example of the code below, but i'm pretty sure it's fine:

 function OnTriggerEnter (trigger : Collider)
 {
 if (trigger.gameObject.tag == "missile"){
 Debug.Log("DESTROY MISSILE BOOOM!!");
 }
 
 if (trigger.gameObject.tag == "roof"){
 Debug.Log("DESTROY ROOF BOOOM!!");
 }
 
 if (trigger.gameObject.tag == "building"){
 Debug.Log("DESTROY BUILDING BOOOM!!");
 }

Thanks, Tom :)