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

Question

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:

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);
    }
}

Does someone have an explanation? Thanks so, so much! :)