Scroll bar isnt scrolling

I tried to input a text from a css, which its height is greater than the target text field, so a scroll bar was placed.

but I was wonder that why the scroll bar doesnt enable scrolling after loading the text.

thanks

import fl.text.TLFTextField;

import fl.controls.UIScrollBar;

var loader:URLLoader = new URLLoader();

var targetXML:XML = new XML();

var txtField:TLFTextField = new TLFTextField();

var scrollBar:UIScrollBar = new UIScrollBar();

txtField.width = 300;

txtField.height = 300;

addChild(txtField);

scrollBar.x = txtField.width + scrollBar.width;

scrollBar.height = txtField.height;

addChild(scrollBar);

loader.load(new URLRequest("http://blahblahblah"));

loader.addEventListener("complete", loadComplete);

function loadComplete(e):void {

    targetXML = XML(loader.data);

    txtField.htmlText = targetXML.channel.item[0].description;

}