Input: Missing mouse button events

I'm trying to create an input system that will catch all left mouse button events.

With the current code I'm missing about 1 in every 35 left mouse button events.

The target platform is a Windows Standalone created in Windows 5 Pro running on a machine with Windows 10. The mouse input is being driven by launching toy ammunition at a touchscreen, at a rate of about 2-ish per second. I'm testing on a scene with no real overhead, just a plane and instantiating a sphere locked at 60 frames per second with vsync. My code is using OnGUI to put mouse events into a queue and Update to process them. I've tried the popular TouchScript and Rewired plug-ins and the result is similar.

Any advice would be greatly appreciated.

 import System.Collections.Generic;
 
 var queue = new List.<Event>();  
 var currentEvent: Event;
 var mouseNumber: int;
 var mouseTextBox: GameObject;
 var mouseSphere: GameObject;
 var mouseVisibleNumber : int; 
 
 
 function OnGUI() {
           currentEvent = Event.current;   
            //find left mouse button clicks
             if (currentEvent.type == EventType.MouseDown && currentEvent.button == 0){
                       //adds to back of queue
                       queue.Add(currentEvent);
                       } 
 }
      
      
 function Update() {
      var hit: RaycastHit;
       //if the queue has anything in it
       if (queue.Count > 0){
            //references front of queue   
             var processingEvent = queue [0];
             var flippingEvent = processingEvent.mousePosition;   
             //flips y coordinate, as putting it in the queue causes it to be inverted somehow
             var flippingY = Screen.height-processingEvent.mousePosition.y;
             var ray = Camera.main.ScreenPointToRay(Vector3(processingEvent.mousePosition.x, flippingY, 0));
                            //if the mouseclick hits something, add number to textbox            
                          if (Physics.Raycast(ray, hit)){
                              mouseNumber ++;   
                              Instantiate (mouseSphere, hit.point, transform.rotation);
                              mouseVisibleNumber ++;
                              var mouseInt = mouseVisibleNumber; 
                              var mouseStr = mouseInt.ToString();
                              mouseTextBox.GetComponent(UI.Text).text = mouseStr;
                                  for (var queuedEvents in queue){
                                  Debug.Log (queue.Count + "," + queuedEvents);
                                  }      
                             queue.RemoveAt(0);
                             }
                             }
                             }