How do i mask a clip.loadMovie(picture.jpg); [FMX2004]

Im loading several thumbnails from out of a database into flash, everything is stored as it should.
Then i create a Rectangle called “mc_mask”.
This one is created as it should with the correct values.
Then im looping through this for(){}.
As you can see im duplicating a movieclip “mc_thumb_holder”
then eval them to my array, there im loading in a Movie with .loadMovie(thumb);
Now when i set the rectangle as a mask on each of the duplicated movieclips it wont work.
So my question is:

How do i mask these duplicatedMovieClips when i did a loadMovie in each of the duplicated Clips?
(all vars and names are doublechecked, everything is loaded and stored as it should, even the thumbs loading… but the mask will not set for some reason, i tried drawing mask as well with layers… but wont work either)

createRectangle(0,0,100,20,“mc_mask”);
thumbNails = new Array();

for (x=0; x<result.items.length; x++) {
duplicateMovieClip(“mc_thumb_holder”, “mc_thumb_holder” add x, x+99);
thumbNails[x] = eval(“mc_thumb_holder” add x);
thumbNails[x].thumbP.loadMovie(thumb);
thumbNails[x].setMask(mc_mask);

}