scripts not working properly drag doesnt work after its used once.

when i use drag so i can move my inventory it doesnt work more then once (i move it once) then it doesnt reset so i can drag again

using UnityEngine; using System.Collections; using System.Collections.Generic;

public class UI : MonoBehaviour { public GUISkin Skin;

 public float lootWindowHeight = 100;
 public float buttonWidth = 40;
 public float buttonHeight = 40;
 public float closeButtonWidth = 20;
 public float closeButtonHeight = 20;

 
 private bool _displayLootWindow = false;
 private float _offset = 10;
 private const int Loot_window_ID = 0;
 private Rect _lootWindowRect = new Rect (0, 0, 0, 0);
 private Vector2 _lootWindowSlider = Vector2.zero;
 public static Interaction interaction;
 
 
 private bool _displayInvWindow = false;
 private const int Inv_window_ID = 1;
 private Rect _invWindowRect = new Rect(10, 10, 170, 265);
 private int _invRows = 5;
 private int _invCols= 5;
 
 private string _tooltip = "";
 
 
 // Use this for initialization
 void Start () {
 }
 
 public void OnEnable() {
     Messenger.AddListener("DisplayLoot", DisplayLoot);
     Messenger.AddListener("CloseIntereaction", ClearWindow);
     Messenger.AddListener("ToggleInv", ToggleInvWindow);

 }
 public void OnDisable() {
     Messenger.RemoveListener("DisplayLoot", DisplayLoot);
     Messenger.RemoveListener("CloseIntereaction", ClearWindow);
     Messenger.RemoveListener("ToggleInv", ToggleInvWindow);

 }
 
 void OnGUI() {
     GUI.skin = Skin;
     
     if(_displayInvWindow)
         _invWindowRect = GUI.Window(Inv_window_ID, _invWindowRect, InvWindow, "Inventory");
     
     if(_displayLootWindow)
         _lootWindowRect = GUI.Window(Loot_window_ID, new Rect(_offset, Screen.height - (_offset + lootWindowHeight), Screen.width - (_offset * 2), lootWindowHeight), LootWindow, "Loot window");
 
     DisplayTooltip();
 }
 
 private void LootWindow(int id) {
     GUI.skin = Skin;
     
     if(GUI.Button(new Rect(_lootWindowRect.width - _offset * 2, 0, closeButtonWidth, closeButtonHeight), "x", " close Window Button")) 
         ClearWindow();
     
     if(interaction == null)
         return;
     
     if(interaction.loot.Count == 0) {
         ClearWindow();
         return;
     }
     
     _lootWindowSlider = GUI.BeginScrollView(new Rect(_offset * .5f, 15, _lootWindowRect.width - _offset, 70), _lootWindowSlider, new Rect(0, 0, (interaction.loot.Count * buttonWidth) + _offset, buttonHeight + _offset));
     
     for(int cnt = 0; cnt <  interaction.loot.Count; cnt++) {
         if(GUI.Button(new Rect(_offset * .5f + (buttonWidth * cnt), _offset, buttonWidth, buttonHeight), new GUIContent(interaction.loot[cnt].Icon, interaction.loot[cnt].ToolTip()))) {
             PlayerCharacter.Inventory.Add(interaction.loot[cnt]);
             interaction.loot.RemoveAt(cnt);
         }
     }
     
     GUI.EndScrollView();
     
     SetToolTip();        
 }
 
 private void DisplayLoot() {
     _displayLootWindow = true;
 }
 
 private void ClearWindow() {
     _displayLootWindow = false;

     
     interaction.OnMouseUp();
     
     interaction = null;
 }
 public void InvWindow(int id) {
     GUI.skin = Skin;
     int cnt = 0; 
     
     for(int y = 0; y < _invRows; y++) {
         for(int x = 0; x < _invCols; x++) {
             if(cnt < PlayerCharacter.Inventory.Count) { 
                 GUI.Button(new Rect(5 + (x * buttonWidth), 20 + (y * buttonHeight), buttonWidth, buttonHeight), new GUIContent(PlayerCharacter.Inventory[cnt].Icon,  PlayerCharacter.Inventory[cnt].ToolTip()), "InvSlotUsed");
             } 
             else {
                 GUI.Label(new Rect(5 + (x * buttonWidth), 20 + (y * buttonHeight), buttonWidth, buttonHeight), (x + y * _invCols).ToString());
             }
             
             cnt++;
         }
     }
     
     SetToolTip();
     GUI.DragWindow();
 }
 
 public void ToggleInvWindow() {
     _displayInvWindow = !_displayInvWindow;
 }
 
 private void SetToolTip() {
     if(Event.current.type == EventType.Repaint && GUI.tooltip != _tooltip) {
         if(_tooltip != "")
             _tooltip = "";
         if(GUI.tooltip != "")
             _tooltip = GUI.tooltip;
         }
     }
 
 private void DisplayTooltip() {
         if(_tooltip != "");
             GUI.Box(new Rect(Screen.width / 2 - 100, 10, 200, 100), GUI.tooltip);
 }

}