Unity2D: OnMouseDrag, OnMouseOver etc. out of a sudden not working for specific GameObject

Hi there,

I am working on a card game. I have a few cards and a few card placeholders. I have created a CardScript to add to each card in the game. I made it so I can drag the card and when it collides with the "PlaceHolder" then it gets pinned at that position as soon as you release the mouse button. This works fine. I am dragging around the cards to placeholders and it looks cool. But then, out of a sudden one or two of the cards do not react at all to my clicking and dragging. I tried logging something in the OnMouseDrag and OnMouseOver event methods, but they do not get fired at all! It's like suddenly the card does not exist anymore for MonoBehaviour. When I drag it around in the scene then suddenly it works again. Here is my CardScript, does someone has an explanation? Thanks so, so much! :)

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class CardScript : MonoBehaviour {
 
     public bool MovementAllowed = true;
     private Vector3 originalPosition;
     private Vector3 placedPosition;
     private Vector3 lastPosition;
     private bool turnedAround = true;
     public Text CardText;
     private Sprite originalSprite;
     private bool isMoving = false;
     public bool mayFlip = true;
 
 
     // Use this for initialization
     void Start () {
         originalPosition = transform.position;
         placedPosition = originalPosition;
         lastPosition = placedPosition;
         originalSprite = gameObject.GetComponent<SpriteRenderer>().sprite;
     }
 
     private void OnTriggerEnter2D(Collider2D collision)
     {
         if (collision.tag == "PlaceHolder")
         {
             ChangePlacedPosition(collision.gameObject.transform.position);
         }
     }
 
     private void OnMouseOver()
     {
         if (mayFlip)
         {
             Debug.Log("Mouse over " + gameObject.name);
             if (Input.GetMouseButtonDown(1))
             {
                 FlipCard();
             }
         }
     }
 
     private void OnMouseDrag()
     {
         if (MovementAllowed)
         {
             if (!isMoving)
             {
                 Debug.Log("Trying to move " + gameObject.name);
             }
             isMoving = true;
             CameraDragScript.mayDrag = false;
             Vector3 followPos = Camera.main.ScreenToWorldPoint(Input.mousePosition);
             transform.position = new Vector3(followPos.x, followPos.y, 0);
         }
     }
 
     private void OnMouseUp()
     {
         isMoving = false;
         CameraDragScript.mayDrag = true;
         transform.position = placedPosition;
     }
 
     public void ChangePlacedPosition(Vector3 position) //Change the position that the card will have when the mouse is released after dragging it around
     {
         lastPosition = placedPosition;
         placedPosition = position;
     }
 
     public void FlipCard()
     {
         if (turnedAround)
         {
             turnedAround = false;
             //Do flip animation, rotate y to -89
             gameObject.GetComponent<SpriteRenderer>().sprite = DataKeeperScript.Instance.EmptyCardSprite;
             PlaceTextOnCard();
             //Do flipover animation, rotate y from -89 back to 0
         }
         else
         {
             turnedAround = true;
             if (CardText != null)
             {
                 Destroy(CardText.gameObject);
             }
             gameObject.GetComponent<SpriteRenderer>().sprite = originalSprite;
         }
         
     }
 
     public void PlaceTextOnCard()
     {
         CardText = Instantiate(UIHandlerScript.Instance.text, GameObject.Find("Canvas").transform);
         CardText.rectTransform.position = Camera.main.WorldToScreenPoint(transform.position);
     }
 }