Next button only works once

programming flash
1

When clicked, the tween works well. but when clicked again nothing happens. why?

if(event.target == btnNext)
{
    promoNum = promoNum+1;
    if(promoNum >= 19) //must change number 19 with XML var
       {
          promoNum = 0;
       }
    trace(promoNum);
    moveEnd = -(((this.getChildByName("promotion"+(promoNum))).width)-100);        
    
    for(var i:Number = 0; i < 19; i++)
    {
    xTween = new Tween(this.getChildByName("promotion"+i),"x",None.easeNone,(this.getChildByName("promotion"+i)).x,moveEnd,1,true);
    trace(moveEnd);
    
    }
}