OnCollisionEnter Problem

Why doesnt the onGround variable turn to true when the player collides with the object that i have tagged as Ground ?

using UnityEngine; using System.Collections;

public class Player : MonoBehaviour {

 public float maxSpeed = 3;
 public float speed = 10f;
 public float jumpPower = 100f;
 
 public bool onGround;
 public bool gotin;
 
 private Rigidbody2D rb2d;
 private Animator anim;

 void Start() 
 {
     rb2d = gameObject.GetComponent<Rigidbody2D>();
     anim = gameObject.GetComponent<Animator>();
 }
 
 
 void Update() 
 {
     //Animator variables
     anim.SetBool ("Grounded", onGround);
     anim.SetFloat("speed",Mathf.Abs(rb2d.velocity.x));

     //Turnimg back the player
     if(Input.GetAxis("Horizontal") < -0.1f){

         transform.localScale = new Vector3(-1,1,1);
     }
     if(Input.GetAxis("Horizontal") > 0.1f){
         
         transform.localScale = new Vector3(1,1,1);
     }
     
 }
 
 void FixedUpdate()
 {

     //Moving the player
     float h = Input.GetAxis ("Horizontal");
     
     rb2d.AddForce (Vector2.right * speed * h);
     
     //Limiting Speed of the player
     if(rb2d.velocity.x > maxSpeed)
     {
         rb2d.velocity = new Vector2(maxSpeed,rb2d.velocity.y);
     }
     
     if(rb2d.velocity.x < -maxSpeed)
     {
         rb2d.AddForce (Vector2.left * speed * h);
         rb2d.velocity = new Vector2(-maxSpeed,rb2d.velocity.y);
     }
     
 }

 //Check if grounded
 void OnCollisionEnter(Collision col)
 {
     if (col.gameObject.tag == "Ground") {
         gotin = true;
         onGround = true;
     }
 }

 void OnCollisionExit(Collision col)
 {
     if(col.gameObject.tag == "Ground")
     {
         onGround = false;
     }
 }

}