Basic collision not working - c#

 void OnCollisionEnter (Collision touch)
     {
         if(touch.gameObject.name == "flag")
         {
             Destroy (touch.gameObject);
         }
     }

So this my code. when the player collides with the flag it should dissapear, however nothing is happening.

Full code: using UnityEngine; using System.Collections;

 public class MovementController : MonoBehaviour {
 
     public float maxSpeed = 10f; //new float variable called maxspeed it equals 10 
     bool facingRight = true; //new bool variable called facingright it equals true
 
     Animator anim;
 
     bool grounded = false;
     public Transform groundCheck;
     float groundRadius = 0.2f;
     public LayerMask WhatIsGround;
     public float jumpForce = 700f;
 
     void Start () 
     {
         anim = GetComponent<Animator>();
     }
     
 
     void FixedUpdate () 
     {
         grounded = Physics2D.OverlapCircle (groundCheck.position, groundRadius, WhatIsGround); //creates circle, groundCheck.position is where the cirle is generated, groundRadius is the radius of the cirle, WhatIsGround is everything it will collide with.
         anim.SetBool ("Ground", grounded); // animation bool is Ground = grounded
         anim.SetFloat ("vspeed", rigidbody2D.velocity.y);
 
         if (!grounded) return;
         float move = Input.GetAxisRaw ("Horizontal"); //how much we are moving
 
         anim.SetFloat ("Speed", Mathf.Abs (move)); //mathf.abs is absoloute value, doesnt matter what direction im moving in.
 
         rigidbody2D.velocity = new Vector2 (move * maxSpeed, rigidbody2D.velocity.y); //move * maxSpeed makes the character move depending on what key they're pressing * maxspeed. rigidbody2D.velocity.y keeps our y value the same.
 
         if (move > 0 && !facingRight) //if moving left and not facing left
             Flip (); //flip
         else if (move < 0 && facingRight) //if moving to the right and not facing right
             Flip (); //flip
     }
 
     void Update()
     {
         //DONT DO THIS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
         if (grounded && Input.GetKeyDown (KeyCode.Space)) // if player is grounded and space is pressed
         {
             anim.SetBool("Ground", false); // set ground to false
             rigidbody2D.AddForce (new Vector2(0, jumpForce));
         }
     }    
 
 
     void Flip ()
     {
         facingRight = ! facingRight; // flips the character
         Vector3 theScale = transform.localScale; //flips the local scale
         theScale.x *= -1; //flip the x axis 
         transform.localScale = theScale; //apply all of this back to the local scale
     }
     void OnCollisionEnter (Collision touch)
     {
         if(touch.gameObject.name == "flag")
         {
             Destroy (touch.gameObject);
         }
     }
 }