Problems positioning inside a movieClip

The _x values of the objects loaded will not update, why am i having such problems changing the values within movies?

CODE:

var speed:Number=1.5;
//
_root.createEmptyMovieClip(‘empty’, this.getNextHighestDepth());
function createSnow(){
for (var i=0;i<20;i++){
_root.empty.createEmptyMovieClip(‘holder_mc’, this.getNextHighestDepth());
_root.empty.holder_mc.attachMovie(‘black_snow’, ‘snow’+i, this.getNextHighestDepth());
var newXPos:Number=Math.floor(Math.random(200)*200);
_root.empty[‘snow’+i]._x=newXPos;
trace(newXPos);
}
}
createSnow();