Targeting motion tweens under masks.....?

Hi–

I have been having a problem since yesterday, which I have finally narrowed down…

I am trying in CS4 to change visibility of some motion tweens using AS2 _visible=true and _visible=false ( I need to stick to AS2)

I am able to successfully target the motion tweens that are not masked, but I can not target the motion tweens that ARE masked. The masks lie directly over the tweens within the same movie clip as the other tweens.

Does anyone know what’s going on here???