Flash AS3 LoaderContext parameters with "undefined" - actionscript-3

Question

I have a main movie clip using loader.
There are two buttons at the child movie clip, That is used for calling the child movie clip with loader.
Main movie clip : main.wf
Initial Loader Movie clip : sub.swf
Button A Click : Call a.swf
Button B Click : Call b.swf
I mean this loader load two movie clip as I click every button.
The main movie clip control passing variables over the child movie clip with LoaderContext.
The problem is that second movie clip(b.swf) cannot get loader context parameters properly.
The error message is that the parameter value is undefined.
// main.swf
var context:LoaderContext = new LoaderContext();
context.parameters = {"qty": qty};
This make me crazy because when I click button A in order to call a.swf(child movie clip)
The parameter get the result correctly in the a.swf.
// a.swf
var qty = this.loaderInfo.parameters.qty; // it's okay at a.swf
However I've finished job in the a.swf that called method which is located in the main.swf.
The method do the job for calling b.swf. The problem happened at this time.
// b.swf
var qty = this.loaderInfo.parameters.qty; // error : undefined value
Please give me an advice to resolve it.
// main.swf
var context:LoaderContext = new LoaderContext();
context.parameters = {"qty": "start"};
var myLoader:Loader = new Loader();
var url:URLRequest = new URLRequest("sub.swf");
myLoader.load(url, context);
addChild(myLoader);
function nextStep0(qty:String):void{
// sub.swf's called this method for updating qty
this.qty = qty;
context.parameters = {"qty": qty};
url = new URLRequest("a.swf");
myLoader.load(url, context);
}
function nextStep1(qty:String):void{
// a.swf's called this method for updating qty
this.qty = qty;
context.parameters = {"qty": qty};
url = new URLRequest("b.swf");
myLoader.load(url, context);
}
This is a sub child what is called initially.
sub.swf
var qty_char = this.loaderInfo.parameters.qty; // "start" get it works
button.addEventListener(MouseEvent.CLICK, clichandle);.... // Event Listener Click
fuction clichandle(e:MouseEvent){
var r_movie:MovieClip = this.parent.parent as MovieClip;
r_movie.nextStep0("alpha");
}
This is a child movie clip
a.swf
var qty_char = this.loaderInfo.parameters.qty; // "alpha" get it works
button.addEventListener(MouseEvent.CLICK, clichandle);.... // Event Listener Click
fuction clichandle(e:MouseEvent){
var r_movie:MovieClip = this.parent.parent as MovieClip;
r_movie.nextStep1("beta");
}
This is a child movie clip too
b.swf
var qty_char = this.loaderInfo.parameters.qty; // error, undefined.
button.addEventListener(MouseEvent.CLICK, clichandle);.... // Event Listener Click
fuction clichandle(e:MouseEvent){
}