hitTestPoint with nested MovieClips

programming flash
1

I having a problem using hittestPoint. I have a movieClip on the stage(back) that contains another movieclip with some constraints(collisions). I add enemy movieclips with their own class(Enemy) to (back) which I want to be contained by (collisions) with hittestpoint. The below code is in the Enemy class where loop is called on enterframe.

The code below works fine and the colission is detected fine(downBumpint = true). The problem I have is that in the game I need to move the movieclip(back) verically. When I do this since(Enemy) is in (back) it’s (Enemy)'s globalPos.y value gets changed(following the movement of (back)) and the hittesPoint is broken returning (downBumping = false).

I guess the question is how do I set it up so that the globalPos stays relative to (back) but also get the hitTestPoint to work?

 _root = MovieClip(root);  // this is defined in Enemy constructor function

// this is looped in Enemy Class
private function loop(event:Event):void
{
var globalPos:Point = this.localToGlobal(new Point());

             if (_root.back.collisions.hitTestPoint(globalPos.x +     downBumpPoint.x ,globalPos.y + downBumpPoint.y ,true))
             {
                 downBumping = true;
             }
             else
             {
                 downBumping = false;
             }
 }