Collision checked once, then never again. How can I make it always check if its touching? C#

I have a function that checks for collisions, I was hoping I could have it change the animation when it sees that I'm not touching or am touching an object. The problem is that it only seems to check the instant I hit play and never again, so I can't change the animation. Is there a way to fix that? The function that is the problem is the last one.

using UnityEngine; using System.Collections;

 public class Player : MonoBehaviour {
 public float speed = 1f;
 public float jumpSpeed;
 public Rigidbody2D rb;
 Animator anim;
 
 void Start () {
     anim = GetComponent<Animator>();
     rb = GetComponent<Rigidbody2D>();
 }
 
 void Update () {
     if (Input.GetKey (KeyCode.D) && (transform.position.x < 3.5)) {
         transform.position += new Vector3 (speed * Time.deltaTime, 0.0f, 0f);
         transform.localScale = new Vector3 (2.25f, 2.25f, 0f);
     }
     
     if (Input.GetKey (KeyCode.A) && (transform.position.x > -3.5)) {
         transform.position -= new Vector3 (speed * Time.deltaTime, 0.0f, 0f);
         transform.localScale = new Vector3 (-2.25f, 2.25f, 0f);
     }
     
     if (Input.GetKeyDown (KeyCode.S)) {
         gameObject.GetComponent<Rigidbody2D> ().gravityScale += 1;
     }
     
     if (Input.GetKeyDown (KeyCode.W)) {
         gameObject.GetComponent<Rigidbody2D> ().gravityScale -= 1;
     }

     if (Input.GetKeyDown (KeyCode.Space)) {
         rb.AddForce(transform.up * jumpSpeed);
     }
 }

 void OnCollisionEnter2D(Collision2D col) {
     if (col.gameObject.name == "Platform01") {
         gameObject.GetComponent<Rigidbody2D> ().constraints = RigidbodyConstraints2D.FreezePosition;
         anim.SetBool ("istouch", true);
         Debug.Log ("True");
     } else {
         anim.SetBool("istouch", false);
         Debug.Log("False");
     }
 }

}