Library Movie clip "visibility" problem

I have a movie clip “target” in my library (and it’s exported for action script)
I the action script attach this to the movie scene and everything works fine

var newTarget = this.attachMovie("target", "target1", depth);

After I’ve added a xml file and moved this part of the code in the xml onload event handler it stopped working.

imagesXML.onLoad = function() {
var newTarget = this.attachMovie("target", "target1", depth);
}

When I try to trace the newTarget object the result is undefined and in the first version the trased message was “_level0.target1” so i suppose that the problem is in the first parameter of the attachMovie method…

Do you have any ideas what’s wrong with my code. Thank you.