how can my player keep jumping wen it hit ground again?

I want to make my player to keep jumping wen it hit the ground a gain if I'm holding my button down, but right know it only jumps one for each time a click or hold the button . is for mobile divice.

this is my script

 public enum type {LeftButton, RightButton, JumpButton,PouseButton};
 public type buttonType;
 
 public float jumpHeight = 0.0f, moveSpeed  = 0.0f;
 
 
 
 
 public GameObject playerObject = null;
 
 
 public GUITexture buttonTexture = null;
 private float myTouchDT = 0;
 private Touch touch;
 private bool isGrounded;
 
 void OnCollisionEnter2D(Collision2D col)
 {
     if (col.gameObject.name == "ground")
         isGrounded = true;
 }
 
 void OnCollisionExit2D(Collision2D col)
 {
     if (col.gameObject.name == "ground")
         isGrounded = false;
 }
 
 
 void OnFirstTouchBegan(){
     
     switch (buttonType)
     {
     case type.JumpButton:
 
         if (playerObject.GetComponent<PlayerManager> ().onGround)
  {
 
             if (myTouchDT < .7f) {
                 playerRigidbody.AddForce (Vector2.up * jumpHeight, ForceMode2D.Impulse); // this make the object jump  // esto checa si esta en el suelo o no pa ver si brinca o no
                 Debug.Log ("small");
             } else if (myTouchDT >= .7f) 
             {
                 playerRigidbody.AddForce (Vector2.up * jumpHeight, ForceMode2D.Impulse); // this make the object jump  // esto checa si esta en el suelo o no pa ver si brinca o no
                 Debug.Log ("keep jumping");
             }
             myTouchDT = 0f;
 
         }
         break;
     }
 }