2d project a shadow below the player

I want to create a shadow in my 2d sidescroller game. Just a round circle that is under the players feet. I want the shadow to get more transparent the higher the player jumps.

so what I did is to cast a raycast 2D from the players feet to the ground and I moved it a bit behind the player to prevent the shadow to appear when the player is hanging on a ledge. It works fine so far but sometimes the shadow just keeps sticking on the players feet while jumping. I don't know why that is that way.

It works again when I reset the scene. Sometimes it works for minutes sometimes just for seconds.

Any idea?

 using UnityEngine;
 using System.Collections;
 using PlatformerPro;
 
 public class ShadowRaycast : MonoBehaviour {
 
     public Transform Shadow;
     public Transform Player;
     public float xOffset;
     private Vector2 raycastOrigin;
     /// <summary>
     /// The character reference.
     /// </summary>
     protected IMob animatable;
     public float opacityGrade;
     public SpriteRenderer shadow;
 
 
     /// <summary>
     /// Unity start hook.
     /// </summary>
     void Start()
     {
         // This is not elegant but its a simple and effective way to handle interfaces in Unity
         animatable = (IMob)gameObject.GetComponentInParent (typeof(IMob));
         if (animatable == null) Debug.LogError ("UnitySpriteDirectionFacer can't find the animatable reference");
         }
 
 
     // Update is called once per frame
     void Update () {
             if (animatable.LastFacedDirection == -1) {
             raycastOrigin = this.transform.position + new Vector3 (xOffset, 0f, 0f);
         }
         else raycastOrigin = this.transform.position + new Vector3 (-xOffset, 0f, 0f);
 
     
         RaycastHit2D hit = Physics2D.Raycast(raycastOrigin, -Vector2.up);
         Vector2 yPosition = hit.point;
         float distance = Mathf.Abs(hit.point.y - this.transform.position.y);
         Shadow.transform.position = yPosition; 
   
         if (distance > 6)
             distance = 6;
         
         opacityGrade = (distance/6-1)*-1;
 
         Debug.Log("The opacityGrade is: "+ opacityGrade);
         shadow.color = new Color(1f, 1f, 1f, opacityGrade);
 
     }
 }