OnCollisionEnter not entered on obvious collision

Hei there

I'm quite new to Unity, and I'm having a problem. I'm spawning an empty casing prefab as part of the fireing of a gun, imparting it with some velocity and letting it fly. I then want to detect when it hits the ground and play a sound and then remove it. But the collision even never fires.

The casing has a capsule collider and a non-kinematic rigidbody on it, and I've double checked that the ground has a (non-trigger) mesh collider on it. The rigidbody starts out being a trigger, but is changed to not be a trigger soon after it's spawned in. When I play the game I can see the casing being spawned and hitting the floor (where it bounces around a bit), but OnCollisionEnter never get's called. Below is the script I've attached to the casing prefab.

 using UnityEngine;
 using System.Collections;
 
 public class CasingScript : MonoBehaviour {
 
     public float timeImmunity;
     public float timeMax;
     float timeCreation;
     bool collided = false;
 
     void Awake() {
         timeCreation = Time.time;
     }
 
     void FixedUpdate() {
         if ((Time.time - timeCreation) > timeImmunity) {
             collider.isTrigger = false;
         }
 
         if (collided && !audio.isPlaying) {
             Debug.Log("Casing destoryed");
             Destroy(gameObject);
         }
 
         if ((Time.time - timeCreation) > timeMax) {
             Debug.Log("Casing timed out");
             Destroy(gameObject);
         }
     }
 
     void OnCollisionEnter(Collision other) {
         if ((Time.time - timeCreation) < timeImmunity) {
             Debug.Log("Casing collided; immune");
             return;
         }
 
         if (!collided) {
             collided = true;
             audio.Play();
             Debug.Log("Casing collided");
         }
     }
     void OnTriggerEnter(Collider other) {
         Debug.Log ("Triggered against " + other.gameObject.name);
     }
 }