Conditionals in UI Drag and Drop?

Hi,
I have a nice UI drag and drop system (big thanks to quill18 and BoredMormon for their tutorials) and want to get some conditionals in it.
I have 4 numbers (that are instantiated on the canvas) that I can to drag and drop on 4 specific dropzones.
The numbers return to their original position if they're not dropped on the specific dropzone.

Here's the Drag script:

 public class Drag : MonoBehaviour, IBeginDragHandler, IDragHandler, IEndDragHandler {    
     public Vector3             positionToReturnTo;
 
     public enum             Slot { ONE, TWO, THREE, FOUR};
     public Slot                typeOfItem = Slot.ONE;
 
     public void OnBeginDrag(PointerEventData eventData){
         positionToReturnTo = this.transform.position;
         GetComponent<CanvasGroup> ().blocksRaycasts = false;
         Debug.Log ("OnBeginDrag");
     }
 
     public void OnDrag(PointerEventData eventData){
         this.transform.position = eventData.position;
         Debug.Log ("OnDrag");
     }
 
     public void OnEndDrag(PointerEventData eventData){
         GetComponent<CanvasGroup> ().blocksRaycasts = true;
         this.transform.position = positionToReturnTo;
         Debug.Log ("OnEndDrag");
     }        
 }  

Here is the Dropzone script:

 public class Dropzone : MonoBehaviour, IDropHandler {
 
     public Drag.Slot     typeOfItem = Drag.Slot.ONE;
 
     public void OnDrop (PointerEventData eventData)
     {
         Debug.Log (eventData.pointerDrag.name + "was dropped on " + gameObject.name);
 
         Drag d = eventData.pointerDrag.GetComponent<Drag> ();
         if (d != null) {
             if (typeOfItem == d.typeOfItem) {
                 d.positionToReturnTo = this.transform.position;
             }        
 
         }
     }
 }

  

The logic I want to put in is:
- First numberOne needs to be placed on dropzoneOne
- Only then numberTwo can be placed on dropzoneTwo
- Only then numberThree can be placed on dropzoneThree
- Only then numberFour can be placed on dropzoneFour

I tried here below with bools but it doesn't work. Can anybody help me?
EDIT:
further information:

Only case “one” does work: numberTwo, numberThree and numberFour bounce back to their original positions before I drop numberOne on dropzoneOne. The bool oneInDropzone is also set to true.

After that: numberTwo, numberThree and numberFour keep bouncing back to their original positions when I drag and drop them. So it stops at case “two”, I can’t drop numberTwo on dropzoneTwo.

I made the bools public and in play mode I see that oneInDropzone = true in the Dropzone script on dropzoneOne, but I don’t see the change on dropZoneTwo,dropZoneThree,dropZoneFour. They all have the same script, how is that possible?

 public class Dropzone : MonoBehaviour, IDropHandler {
 
     public Drag.Slot     typeOfItem = Drag.Slot.ONE;
 
     bool                 oneInDropzone = false; 
     bool                 twoInDropzone = false; 
     bool                 threeInDropzone = false; 
 
 
     public void OnDrop (PointerEventData eventData) {
         Debug.Log (eventData.pointerDrag.name + "was dropped on " + gameObject.name);
 
         Drag d = eventData.pointerDrag.GetComponent<Drag> ();
         if (d != null) {
             if (typeOfItem == d.typeOfItem) {
                 switch (eventData.pointerDrag.tag) {
                 case "one":
                     d.positionToReturnTo = this.transform.position;
                     oneInDropzone = true;
                     break;
                 case "two":
                     if (oneInDropzone == true) {
                         d.positionToReturnTo = this.transform.position;
                         twoInDropzone = true;
                     }
                     break;
                 case "three":
                     if (twoInDropzone == true) {
                         d.positionToReturnTo = this.transform.position;
                         threeInDropzone = true;
                     }
                     break;
                 case "four":
                     if (threeInDropzone == true) {
                         d.positionToReturnTo = this.transform.position;
                     }
                     break;
                 }
             }
                 
         }
 
     }
 
 }