OnTriggerEnter2D Not Working

I have a shot prefab with rigidbody 2d and box collider 2d components that is set to be a trigger. However, OnTriggerEnter2D is not set off when it collides with either the player, which also has a rigidbody 2d and box collider 2d, or with an enemy, which has a box collider 2d. None of the objects are kinematic, and they are all on the same layer. Here is the code for collision detection:

 void OnTriggerEnter2D(Collider2D collider)
     {
         Debug.Log ("Entered");
         ShotScript shot = collider.gameObject.GetComponent<ShotScript>();
         if (shot != null) {
             if (shot.enemyShot != isEnemy) {
                 hp -= shot.damage;
                 Destroy (shot.gameObject);
             }
 
             if (hp <= 0) {
                 Destroy (gameObject);
             }
         } 
     }

Here is the shot prefab:

Here is the enemy prefab (the health script component is the code above):