2D Drag and Drop, but not when slot is already full

Hi,

First time asking for help, I'm very new to Unity but I've managed to create 90% of a game so far. I'm on the last puzzle and I can't get my head around this issue.

Basically, I have 35 objects and 3 slots to drop the objects on to. If there is already an object in a slot, it should be considered that the slot is busy and if the user tries to drop a second object onto an occupied slot, it should snap back to it's original location. The script I have deals with the drag and drop and the snapping etc, but I can't figure out how to stop the user dropping multiple items into the same slot!

Here's a screenshot, the code follows:

Code:

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class DragViles : MonoBehaviour
 {
     private bool isDragging;
     bool isInSection1, isInSection2, isInSection3, isInShelfArea, section1busy, section2busy, section3busy;
     float old_z;
     bool section1Full = false;
     public void OnMouseDown()
     {
         old_z = this.transform.position.z;
         this.transform.position = new Vector3(this.transform.position.x, this.transform.position.y, -1.671000f);
         isDragging = true;
 
     }
 
     public void OnMouseUp()
     {
 
         this.transform.position = new Vector3(this.transform.position.x, this.transform.position.y, old_z);
 
         isDragging = false;
 
         if (this.transform.position.x == GameObject.Find("position_1").GetComponent<BoxCollider2D>().transform.position.x && this.transform.position.y == GameObject.Find("position_1").GetComponent<BoxCollider2D>().transform.position.y)
         {
             section1Full = true;
         }
 
     }
     private void OnTriggerEnter2D(Collider2D collision)
     {
         if (collision.gameObject.name == "position_1")
         {
             isInSection1 = true;
             isInSection2 = false;
             isInSection3 = false;
             isInShelfArea = false;
             Debug.Log("Position 1");
 
         }
         else if (collision.gameObject.name == "position_2")
         {
             isInSection1 = false;
             isInSection2 = true;
             isInSection3 = false;
             isInShelfArea = false;
             Debug.Log("Position 2");
         }
         else if (collision.gameObject.name == "position_3")
         {
             isInSection1 = false;
             isInSection2 = false;
             isInSection3 = true;
             isInShelfArea = false;
             Debug.Log("Position 3");
         }
         else if (collision.gameObject.name == "shelf")
         {
             isInSection1 = false;
             isInSection2 = false;
             isInSection3 = false;
             isInShelfArea = true;
             Debug.Log("shelf");
         }
 
     }
 
     // Update is called once per frame
     void Update()
     {
         if (isDragging)
         {
 
             Vector2 mousePosition = Camera.main.ScreenToWorldPoint(Input.mousePosition) - transform.position;
             transform.Translate(mousePosition);
 
         }
         else
         {
 
             if (isInSection1)
             {
                 Debug.Log(section1Full);
                 if (section1Full == true)
                 {
                     transform.position = new Vector3(GameObject.Find(this.name + "_position").GetComponent<BoxCollider2D>().transform.position.x, GameObject.Find(this.name + "_position").GetComponent<BoxCollider2D>().transform.position.y, old_z ); ;
                     return;
                 }
                 else
                 {
                     transform.position = new Vector3(GameObject.Find("position_1").GetComponent<BoxCollider2D>().transform.position.x, GameObject.Find("position_1").GetComponent<BoxCollider2D>().transform.position.y, this.transform.position.z); ;
                     return;
                 }
 
             }
             else if (isInSection2)
             {
                 transform.position = new Vector3(GameObject.Find("position_2").GetComponent<BoxCollider2D>().transform.position.x, GameObject.Find("position_2").GetComponent<BoxCollider2D>().transform.position.y, this.transform.position.z);
 
             }
             else if (isInSection3)
             {
                 transform.position = new Vector3(GameObject.Find("position_3").GetComponent<BoxCollider2D>().transform.position.x, GameObject.Find("position_3").GetComponent<BoxCollider2D>().transform.position.y, this.transform.position.z);
 
             }
             else if (isInShelfArea)
             {
                 transform.position = new Vector3(GameObject.Find(this.name + "_position").GetComponent<BoxCollider2D>().transform.position.x, GameObject.Find(this.name + "_position").GetComponent<BoxCollider2D>().transform.position.y, this.transform.position.z);
 
             }
 
         }
 
     }
 }