ActionScript 3.0 :: ROLL_OVER Doesn't Trigger Until Mouse Moves?

I'm seeing inconsistent ROLL_OVER behavior. I have two movieclips (A and B) both containing an alpha=0 hit area child mc (rectangle contained in a mc symbol with the symbol's alpha set to 0 in the parent). I add the same event listener in each parent A & B:

hit.addEventListener(MouseEvent.ROLL_OVER, onRollOver, false, 0, true);
function onRollOver(e:MouseEvent):void {
trace("Mouse Rolled Over");
}

In parent A, when the cursor is over the hit area as the clip appears (using TweenLite.to(alpha), the listener above triggers properly without moving the cursor. In parent B, I have to move the curosor in order for the above listener to trigger. Moreover, in parent B, when the eventlistener is attached to the hit child mc (as shown above) rather than "this" (parent B), I actually have to remove the cursor from the hit area entirely and re-enter the hit area for the above listener to trigger? The parent mc's are loaded differently. Parent A, which works more reliably (mouse doesn't need to move in order for the listener to trigger), is pre-loaded and made visible (.visible = true). Parent B is rendered on the fly in the timeline and is a child of three levels of parents/grandparents...