[CS3] Dynamic transition (mask) not working??

[flashpipe1]

I've got a clip, clipPH2_mc, in a layer. Above that, I've got another clip, clipPH1_mc, and above that, I've got another layer with a placeholder clip, transPH1_mc. In my library, I have a movieclip which is a shape animation, with a linkage name, trans1.

What I'm trying to do is dynamically make the transition mask clipPH1_mc to reveal it, but it isn't working...any ideas??

Here's the code:

transPH1_mc.attachMovie("trans1","transPH1_mc");
clipPH1_mc.setMask("transPH1_mc");

And I attached the clip...

THANKS!!!