setProperty(_root.firm1, _alpha, 0); does not work

Hello everyone!
I have tried to solve a problem for a week or so. Perhaps someone here can help me.
setProperty(_root.firm1, _alpha, 0); does not work in a swf which I have loaded by loadMovie into a main-film. It works perfectly on it´s own if I play it in a own seperate window. What can I do to solve this problem which is a huge problem while I hav many buttons which shall be transparent.
Mvh//Ditti