Destroy enemys head if hit by a bullet? ( Child Collision ) are my component settings wrong?

Hi I am using the Zombie model from the asset store and I am trying to add an OnTriggerEnter script to its head but I have had no luck it simply does not register as a hit. but if I do it with the main body it works.

I have seen many people posting about it but none of the sollutions have worked for me. Is it just a problem with my component settings? ridgidbody/no ridgid body I feel like I have tried every combination.