Loop doesn't position mcs in right place

1

i have a loop as follows and the positioning is doing nothing, they are all just standing in one place one on top of the other. any help is greatly appreciated.

this.createEmptyMovieClip("thumbHolder_mc", this.getNextHighestDepth());
for (var i:Number = 0; i < 20; i++) {
    var thumbnail = "thumb" + i + "_mc";
    thumbHolder_mc.attachMovie("thumb", "thumbnail", 1000 + i);
    thumbnail._x += (i * 10);
}