Can't assign .onPress to a movieclip?

I have a moveclip that is being masked, and is buried inside several other movie clips. One of the movie clips that it is buried in has an .onRollover attached to it.

What I want to do with this clip is put an .onPress on it. However, it seems that I can’t.

Can you not put an .onPress on a movie clip that is masked?
Can you not put an .onPress on a movie clip that is in another movie clip with an .onRollOver on it?

I’m confused, never had this problem before?

-gl