OnCollisionEnter not being called, between a Rigidbody and a Box Collider

Hi Guys,

For the love of me I can't figure this out, I've been searching around the forums like crazy too. Everything I read seems like it should work, however OnCollisionEnter never gets called.

I have an instantiated Projectile with a Rigidbody attached to it, this Projectile has a script on it with the OnCollisionEnter method. This script inherits from MonoBehaviour.

I have 3 cubes with Box Colliders on each of them.

The Projectile simply passes straight through them, and there is no OnCollisonEnter method being called. Here is my code, I have included the entire script as it is relatively short, even though I doubt 99% of it will be of importance.

 using UnityEngine;
 using System.Collections;
 using System.Collections.Generic;
 
 public class ProjectileBehaviour : MonoBehaviour {
 
     private List<Effect> effects;
     private float distance;
     private Vector3 start;
 
     public void activate(List<Effect> e, Material m, float d){
         this.effects = e;
         this.renderer.material = m;
         this.distance = d;
 
         start = transform.position;
 
         Debug.Log ("activated!!");
 
     }
 
     void Update(){
         if(Vector3.Distance (start, transform.position) > distance){
             Destroy (gameObject);
         }
 
     }
 
     void OnCollisionEnter(Collision other){
         Debug.Log ("collision!!");
         UnitVars targetVars = other.gameObject.GetComponent<UnitVars>();
         foreach(Effect e in effects){
             e.apply (targetVars);
         }
         Destroy (gameObject);
 
     }
 }