Duplicated MovieClips disappearing during play

Question

am making my own version of a spaceship fighting game. At one point I want to randomly spawn enemies. Here is what I am doing.
I have a movieclip in the library called enemyShip, AS-Linkage is normalEnemy. Object not placed on the scene. On the actionScipt on that frame, I have the code bellow, which does that for me. What is happening is this: when I publish and run the swf file, the first two enemies spwan properly, starting from the third each new spawn 'hides', but does not delete, the one before it. the second enemy is the first to disappear. The first enemy stays visible. Ideas please? I have tried duplicateMovieClip and attachMovieClip and they both have the same result. Sorry for the long question!
var origEnemy:MovieClip = _root.attachMovie("normalEnemy", "origEnemy",  this.getNextHighestDepth();
var numberOfEnemies:Number = 1;
var enemyIndex:Number = 0;
_root.onLoad();
  spawnEnemy();
  interval = setInterval(spawnEnemy,1000 + Math.random() * 2000);
// Spawns new enemies
function spawnEnemy()
   if (numberOfEnemies > 10)
  clearInterval(interval);
   // Randomly calculate the starting coordinates of newly spawned enemy 
   var x_position = Math.random() * 50;
   var y_position = Math.random() * 700;
   // spawn a new enemy and assign random location 
  enemiesArray.push(origEnemy.duplicateMovieClip("enemy" + enemyIndex, this.getNextHighestDepth()));
   //_root.attachMovie("normalEnemy","enemy" + enemyIndex,this.getNextHighestDepth());
   // assign random location
  enemiesArray[enemyIndex]._x = x_position;
  enemiesArray[enemyIndex]._y = y_position;
   // incerement number of enemies
  numberOfEnemies++;
   // Increment enemy Index
  enemyIndex++;

Answer 1

if 'this' is not the same as _root, the following line is problematic:
var origEnemy:MovieClip = _root.attachMovie("normalEnemy", "origEnemy",  this.getNextHighestDepth();
ie, use:
var origEnemy:MovieClip = _root.attachMovie("normalEnemy", "origEnemy",  _root.getNextHighestDepth();