Strange errors accessing asset stored in swf file

I have a movieClip instance named belt_lines_still added to another movieClip named mainBkgnd, all stored in a swf file. Even though I can access all the other children of mainBkgnd, for some reason I can’t access belt_lines_still.

To diagnose the problem I used these lines:

trace(“childIndex=”+mainBkgnd.getChildIndex(belt_lines_still)+" "+(belt_lines_still!=null));

That returned:

childIndex=-1 false

Can someone explain what’s going on?

What does this do?

var clip = mainBkgnd.getChildAt(3);
trace(clip.parent == mainBkgnd);

That gets the “child display object instance that exists at the specified index” in this case 3. I just used those lines to show that mainBknd does have a child named belt_lines_still. mainBkgnd.getChildIndex(clip)) should return 3 instead -1 which means that mainBkgnd has no child named belt_lines_still.