Opacity effects on Rollover

Has anyone come across this before?

I have a movie clip for a form that i have placed on the stage. The rollover actions for my buttons are that i have turned the opacity (alpha) down to 40% - so when you rollover the button it allows the background to show through.

However, when you test the movie and rollover the button, the opacity effect doesn’t work. Any other rollover effect seems to work, such as changing the colour of the button for example.

Instead of having the form as a movie clip and placed on the stage and just had it built on the stage from scratch, the effect works on the button.

Is this because its in a movie clip or am i completely missing something?