Can't access alpha

I can’t seem to access the alpha property of a movie clip. I can change it’s y x position but not the alpha…

This works great:
loader2.lettercontainer.letter_mc1._y=50;

But this doesn’t do anything:
loader2.lettercontainer.letter_mc1._alpha=50;

The movieclip “letter_mc1” contains a createTextField with one letter in it.
I’m completely lost why this isn’t working…