Bullets sometimes flying through enemies in Unity2D

I'm making a simple unity 2d space shooter game for iPhone where the player has two virtual joysticks, one for movement and one for shooting. I noticed that when I shoot at an enemy while moving away from it above a certain speed the bullets fail to hit the enemy. I have rigidbody2d attached to both the bullet and the enemy. Both have colliders attached, the one on the bullet is a trigger. Here are my codes for the bullet:

 #pragma strict
 var EnemyBulletDelay:float;
 var BulletDamage:int;
 var Spark:GameObject;
 
 function Start () {
     //destroy bullet after delay
     Destroy(this.gameObject,1+EnemyBulletDelay);
 }
 
 function OnTriggerEnter2D(other:Collider2D){
     //detect if enemy taking player bullet shots or player taking enemy bullet shots
     if ((other.gameObject.tag=="Enemy"&&gameObject.tag=="Bullet")||(other.gameObject.tag=="Player"&&gameObject.tag=="EnemyBullet")){
     //target takes damage
         other.SendMessage("takeDamage",BulletDamage);
     //activates spark particles on hit
         if(Spark){
             Instantiate(Spark,transform.position,transform.rotation);
         }
     //destroy bullet
         Destroy(this.gameObject);
     }
 }

And the codes for enemy:

 #pragma strict
 
 var EnemyHealth:int;
 var GameManager:GameObject;
 var ScoreWorth:int;
 var Explosion:GameObject;
 
 function Start(){
     GameManager=GameObject.Find("GameManager");
 }
 
 function takeDamage(damage:int){
     if (EnemyHealth!=0){
         if(EnemyHealth-damage>0){
                 StartCoroutine("flashRed");
         }
         EnemyHealth-=damage;
         if (EnemyHealth<=0){
             EnemyHealth=0;
             DeathSequence();
         }
     }
 }
 
 function flashRed(){
     renderer.material.color=Color.red;
     yield WaitForSeconds(.1);
     renderer.material.color=Color.white;
 }
 
 function DeathSequence(){
     SendMessage("Dead");
     GameManager.SendMessage("AddScore",ScoreWorth);
     Instantiate(Explosion,transform.position,transform.rotation);
     Destroy(this.gameObject);
 }

Please help!