OnTriggerEnter and -Exit not getting called

I'm trying to make text appear if the player is inside a trigger and presses 't', but OnTriggerEnter doesn't get called when I enter the trigger. I've double checked the layers, tags, if the player has a rigidbody and if the collider is set to trigger but I can't find the problem. I'm at my wits' end.

Also, my game is in 2D in case that makes any difference.

 public int conStage = 1;
 public bool trigger;

 void OnTriggerEnter (Collider other){
     if (other.gameObject.tag == "Player") {
         trigger = true;
     }
 }

 void OnTriggerExit (Collider other){
     if (other.gameObject.tag == "Player") {
         trigger = false;
     }
 }



 void Update (){

     if (Input.GetKeyDown (KeyCode.T) && trigger == true) {
         conStage = conStage + 1;
     }

 }

}