[F8] Can This Even Work?

I'm trying to apply a gradient alpha mask to loaded movie clips, the clip and the mask must be cachedAsBitmap for this effect to be achieved according to what I've read. The problem is it works if I use AttachMovie instead of LoadMovie, but attachMovie isn't an option unfortunately, can anyone see where I may be going wrong?

The Working AttachMovie code
_root["container"+n].createEmptyMovieClip("mcontainer"+n, this.getNextHighestDepth());
_root["container"+n]["image"+i].attachMovie("us", "us"+i, this.getNextHighestDepth())
_root["container"+n]["image"+i]["us"+i].cacheAsBitmap = true;
_root["container"+n]["mcontainer"+n].attachMovie("mask", "mask"+n, this.getNextHighestDepth());
_root["container"+n]["mcontainer"+n]["mask"+n].cacheAsBitmap = true
_root["container"+n]["image"+i]["us"+i].setMask(_root["container"+n]["mcontainer"+n]["mask"+n]);


The Not Working LoadMovie Code
_root["container"+n].createEmptyMovieClip("mcontainer"+n, this.getNextHighestDepth());
_root["container"+n]["image"+i].loadMovie(imageurl)
_root["container"+n]["image"+i].cacheAsBitmap = true;
_root["container"+n]["mcontainer"+n].attachMovie("mask", "mask"+n, this.getNextHighestDepth());
_root["container"+n]["mcontainer"+n]["mask"+n].cacheAsBitmap = true
_root["container"+n]["image"+i].setMask(_root["container"+n]["mcontainer"+n]["mask"+n]);


They are practically identical but the mask wont set for the LoadMovie clip and just displays as a graphic above the loaded image... any ideas?!