OnCollisionEnter not called.

I have set up a simple system with a ball (with an attached rigidbody) falling by gravity onto a position locked box (which also has a rigidbody), everything is in the right layers and the physics works correctly. However onCollisionEnter is never called.

Everything has a rigidbody, nothing is marked as a trigger. Both have colliders.

Neither of the objects are kinematic.

Even if I remove constraints on the box or sphere no event is called.

All objects are in the same layer that has physics /collision detection enabled.

It is particularly bizarre as the box /sphere will react to hitting each other, the physics still works but the event is not called.

Can anyone help me work out why it may not be called?

the “O” in oncollisionenter needs to be uppercase

void OnCollisionEnter(Collision col)
{
   Debug.Log("I'm working");
}