mouseUp unreliable on iOS (see code)

About half the time, I find buttons using the code below get stuck in the "down" state. There is no relible factor - sometimes the MOUSE_UP works and sometimes it doesn't. What am I doing wrong here?? (This is on iOS, BTW)

Any thoughts?

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package  {

          import flash.display.MovieClip;

          import flash.events.MouseEvent;

          import flash.events.Event;

          public class buttons extends MovieClip {

                    public function buttons()

                   {

                              this.gotoAndStop(1);

                              this.addEventListener(MouseEvent.MOUSE_DOWN, downPress);

                              this.mouseChildren = false;

                    }

                    private function downPress(event:MouseEvent):void

                    {

                //frame 4 in my button mc holds the down btn graphic

                              this.gotoAndStop(4);

                              this.addEventListener(MouseEvent.MOUSE_UP, upChuck);

                              this.addEventListener(MouseEvent.MOUSE_OUT, rollingOut);

                              this.removeEventListener(MouseEvent.MOUSE_DOWN, downPress);

                    }

                    private function rollingOut(event:MouseEvent):void

                    {

                              this.gotoAndStop(1);

                              this.addEventListener(MouseEvent.MOUSE_DOWN, downPress);

                              this.removeEventListener(MouseEvent.MOUSE_OUT, rollingOut);

                              this.removeEventListener(MouseEvent.MOUSE_UP, upChuck);

                    }

                    private function upChuck(event:MouseEvent):void

                    {

                              this.gotoAndStop(1);

                              this.removeEventListener(MouseEvent.MOUSE_UP, upChuck);

                              this.removeEventListener(MouseEvent.MOUSE_OUT, rollingOut);

                              this.addEventListener(MouseEvent.MOUSE_DOWN, downPress);

                              trace("up");

                    }

          }

}