Why is my OnTriggerEnter not working ?

I earlier had OnTriggerEnter2D in my code along with a Box Collider 2D and a Rigidbody2D attached but I had to change some things and change the those to their 3d counterparts. I removed the BoxCollider2D and Rigidbody2D and edited the code to have OnTriggerEnter instead of OnTriggerEnter2D. Now suddenly, I have no collisions that get detected in the game. Nothing gets debugged either. I have ‘IsTrigger’ ticked on, on the BoxCollider and ‘IsKinematic’ ticked off, on the Rigidbody. Why are no triggers being detected ? The following is my code:

void OnTriggerEnter(Collider kaboom)
		Debug.Log ("colliding");
		if (!invulnerable) {
			if (kaboom.gameObject.tag == "Crowd") {
				invulnerable = true;                    // disallow further damage
				playerDamageByCollision = true;				

PS: I’m using the 2D mode in Unity

If you are using 2D sprites and 2D physics you need a 2D trigger enter callback

2D and 3D physics are completely separate systems.