TOUCH_TAP not firing sometimes on iPhone

Hi,

I find that when I have sprites on the screen, the TOUCH_TAP event sometimes does not fire.

In the following program I generate random squares.  On each tap event, I place a circle on where the tap occured.  When there are little or no squares, the tap event is always triggered and the circle is placed below the tap.  However, as I increase the number of squares, the probability of the tap event not firing increases.  Here I only have 5 squares, and the tap event is not fired 1 out of 50 taps.  As I increase it to 100, the probablity increaes to 1 out of 10.

package

{

  import flash.desktop.NativeApplication;

  import flash.events.Event;

  import flash.display.Sprite;

  import flash.display.StageAlign;

  import flash.display.StageScaleMode;

  import flash.events.TouchEvent;

  import flash.ui.Multitouch;

  import flash.ui.MultitouchInputMode;

  import flash.display.StageOrientation;

  /**

   * ...

   * @7111211

   */

  public class Main extends Sprite

  {

    public var m_clickPos:Sprite; // sprite that tells where you clicked

    public var m_squares:Sprite; // parent sprite that contains random squares

    public const k_numSquares:int = 5;

    public function Main():void

    {

      stage.scaleMode = StageScaleMode.NO_SCALE;

      stage.align = StageAlign.TOP_LEFT;

      stage.addEventListener(Event.DEACTIVATE, deactivate);

      // touch or gesture?

      Multitouch.inputMode = MultitouchInputMode.TOUCH_POINT;

      stage.setOrientation(StageOrientation.ROTATED_LEFT);

      stage.addEventListener(TouchEvent.TOUCH_TAP, onTap);

      m_squares = new Sprite();

      genSquares();

      addChild(m_squares);

      m_clickPos = new Sprite();

      m_clickPos.graphics.beginFill(0xFFFFFF);

      m_clickPos.graphics.drawCircle(0, 0, 5);

      m_clickPos.graphics.endFill();

      m_clickPos.graphics.beginFill(0);

      m_clickPos.graphics.drawCircle(0, 0, 1);

      m_clickPos.graphics.endFill();

      addChild(m_clickPos);

    }

    private function deactivate(e:Event):void

    {

      // auto-close

      NativeApplication.nativeApplication.exit();

    }

    public function onTap(event:TouchEvent):void

    {

      m_clickPos.x = event.stageX;

      m_clickPos.y = event.stageY;

    }

    public function randomInt(low:uint = 0, high:uint = 0):int

    {

      return Math.floor(Math.random() * (1 + high - low)) + low;

    }

    public function genSquares():void

    {

      var sprite:Sprite;

      var squareWidth:int = 30;

      var colors:Vector.<uint> = new Vector.<uint>;

      colors.push(0xFF0000);

      colors.push(0xFFFF00);

      colors.push(0x00FF00);

      colors.push(0x0000FF);

      for (var square:int = 0; square < k_numSquares; square++)

      {

        sprite = new Sprite();

        sprite.x = randomInt(0, 960 - squareWidth);

        sprite.y = randomInt(0, 640 - squareWidth);

        sprite.graphics.beginFill(colors[randomInt(0, colors.length - 1)]);

        sprite.graphics.drawRect(0, 0, squareWidth, squareWidth);

        sprite.graphics.endFill();

        m_squares.addChild(sprite);

      }

    }

  }

}

I am using iPod Touch.  Any ideas why this is happening and how to work around this?

Thanks