ActionScript 3.0 :: Adding Content To A Loaded Movie

I've got an application which loads anexternal swf, then needs to add content to a specific depth in it. It does this by finding a named placeholder clip and replacing it with a UIComponent, using addChildAt(); This seems to work fine - I have a trace utility that walks back up the display list and it shows my UIComponent right where I expect it, then the parent movie clip, then all the containers up to the stage. At each level the container is a) visible b) 1.0 alpha and c) the size I expect it to be, and from my trace I can see that I have a structure which looks something like this:[code]

All well and good, but if I draw a rectangle on the graphics of the UIComponent (or add an image to it, or do anything else), nothing is visible. I can trace any parameters of the UIComponent, I can use it to make changes to its parent SWF, I can even note that if I draw a rectangle on the UIComponent which is larger than the Loaded SWF, then the SWF changes size to accommodate the new bounds - but I can't see a thing.I was wondering if some security stopped the parent application adding content to the Loaded SWF, or if there was some other issue.