Help with attachMovie()

Can some one tell me why the following code won’t display the new instance’s position and instead report “undefined”?

row = column = 0;
for (i=0; i<30; i++) {
[indent]for (j=0; j<20; j++) {[indent]newNoc = "noc_" + row + "_" + column;

positionX = j*(66.3+10);
positionY = i*(41.1+5);
depth = this.getNextHighestDepth();
this.attachMovie("noc", newNoc, depth, {_x:positionX, _y:positionY});
 
// this one reports that the instance is there
trace(this.getInstanceAtDepth(depth)); 
 
//this one gives undefined
trace(this.newNoc._x); 
column++;

[/indent]}

row++;
column=0;

[/indent]}

stop();