Why did the y value stay the same?

I am trying to detect if the player is falling by using the negative velocity.y value.

The y value in the following line of code should change continuously if the player is jumping up and down, but the y value changed for a bit and stayed the same value when the player was jumping up and down. I don’t know why.

Debug.Log (“y =” + rigidbody2D.velocity.y);

       using UnityEngine;
        using System.Collections;
    
        public class Player : MonoBehaviour {
    
            //player movement
            public float movementSpeed = 5.0f;
    
            private int jumpHeight = 250;
            private bool isGrounded = false;
    
        void Update () {
                Debug.Log ("y =" + rigidbody2D.velocity.y);
                if (isGrounded) {
                    if (Input.GetButtonDown ("Jump") || Input.GetMouseButtonDown(0))    {
                        Jump ();
                    }
                }
        }
        void Jump(){ 
               if (!isGrounded) { return; 
               } 
               rigidbody2D.AddForce (new Vector2 (0, jumpHeight), ForceMode2D.Force); 
    }
    
    void OnCollisionEnter2D(Collision2D collision) 
        {
            isGrounded = true;
            Jump ();
        }
    void OnCollisionExit2D(Collision2D collision){
    		isGrounded = false;
    	}
}

The velocity is changing, but you are seeing the most common velocity in the Console.
Open the console and you will see it is still changing or add this code:

void OnGUI ()
{   
     GUILayout.Label("y =" + rigidbody2D.velocity.y);    
}