Need help with a script execution fault

I have a script execution problem. Basically in the following attached code, the script works perfectly at the starting first usage iteration. But then when I repeat the process the second time, the value of _isSelecting variable is not updated, as I see it in the Inspector Window. I need to see if the fault is in my scripting, or is there a problem in the compiler.

 using UnityEngine.EventSystems;
 using UnityEngine;
 using System.Collections.Generic;
 
 public class EngineClickHandlerModule : MonoBehaviour {
 
     // Access different modules of Engine
     EngineSelectionModule _sm;
     EngineUnitFormationModule _ufm;
     EngineBuildingPlacementModule _blpm;
 
     [SerializeField]
     List<GameObject> selectedObjects = new List<GameObject>();
 
     [SerializeField]
     bool _isSelecting = false;
 
     Vector3 _mousePosition1;
     Vector3 _mousePosition2;
 
     // Use this for initialization
     private void Awake()
     {
         _sm = GameObject.Find("SelectionManager").GetComponent<EngineSelectionModule>();
         _ufm = GameObject.Find("UnitFormationManager").GetComponent<EngineUnitFormationModule>();
     }
 
     // Update is called once per frame
     void Update () {
 
         if (EventSystem.current.IsPointerOverGameObject())
         {
             _isSelecting = false;
             return;
         }
 
 
         if (Input.GetMouseButton(0))
         {
             if (_isSelecting == false)
             {
                 Debug.Log("Clicking");
                 foreach(var selectableObject in FindObjectsOfType<UnitSystems>())
                 {
                     if (!IsWithinSelectionBounds(selectableObject.gameObject))
                     {
                         selectableObject.transform.root.Find("SelectionCircleHalo").gameObject.SetActive(false);
                     }                    
                 }
                 selectedObjects.Clear();
                 _sm.ResetUnitSelection();
                 _mousePosition1 = Input.mousePosition;
                 _isSelecting = true;
             }            
         }
         else
         {
             _isSelecting = false;
         }
 
         if (_isSelecting)
         {
             foreach(var selectableObject in FindObjectsOfType<UnitSystems>())
             {
                 if (IsWithinSelectionBounds(selectableObject.transform.root.gameObject))
                 {
                     if (!selectedObjects.Contains(selectableObject.transform.root.gameObject))
                     {
                         selectableObject.transform.root.Find("SelectionCircleHalo").gameObject.SetActive(true);
                         selectedObjects.Add(selectableObject.transform.root.gameObject);
                     }                    
                 }
             }
         }
 
         if (Input.GetMouseButtonUp(0))
         {
             _mousePosition2 = Input.mousePosition;
             if(_mousePosition1 == _mousePosition2)
             {
                 ClickOperations(0);
             }
             else
             {
                 _sm.SetMultipleSelectedUnitsList(selectedObjects);
             }
         }
 
 
 
         if (Input.GetMouseButtonDown(1))
         {
             _isSelecting = false;
             if (_sm._areMultipleUnitsSelected)
             {
                 ClickOperations(3);
             }
             else
             {
                 ClickOperations(1);
             }            
         }
     }
 
     void ClickOperations(int caseNo)
     {
         RaycastHit hitInfo = new RaycastHit();
         bool _hit = Physics.Raycast(Camera.main.ScreenPointToRay(Input.mousePosition), out hitInfo);
 
         if (_hit)
         {
             switch (caseNo)
             {
                 case 0:
                     _sm.CheckOnLeftSingleClickConditions(hitInfo);
                     break;
 
                 case 1:
                     _sm.CheckOnRightSingleClickConditions(hitInfo);
                     break;
 
                 case 3:
                     _sm.CheckOnRightMultipleClickConditions(hitInfo);
                     break;
             }
         }
     }
 
     public List<GameObject> GetSelectedUnits()
     {
         return selectedObjects;
     }
 
     public bool IsWithinSelectionBounds(GameObject gameObject)
     {
         if (!_isSelecting)
             return false;
 
         var camera = Camera.main;
         var viewportBounds = Utils.GetViewportBounds(camera, _mousePosition1, Input.mousePosition);
         return viewportBounds.Contains(camera.WorldToViewportPoint(gameObject.transform.position));
     }
 
     void OnGUI()
     {
         if (_isSelecting)
         {
             var rect = Utils.GetScreenRect(_mousePosition1, Input.mousePosition);
             Utils.DrawScreenRect(rect, new Color(0.8f, 0.8f, 0.95f, 0.25f));
             Utils.DrawScreenRectBorder(rect, 2, new Color(0.8f, 0.8f, 0.95f));
         }
     }
 
 }