Primatte Keyer hell

Hi Gang-

I am having a completely stupid, yet debilitating, problem when trying to use Primatte Keyer 4.1 in AE CS5 on my Mac. Whenever I try to perform a sampling, foreground or background, AE instead starts moving the position of the layer I'm keying. I've tried every possible combination of selecting and/or deselecting the Effect name, the Comp window, the Timeline window. I've also restarted AE, restarted my computer and trashed AE's preferences. It just seems as though it's stuck in a rut.

One useful piece of information (maybe). If I start a new project with a fresh clip and then apply Primatte, it works.

Anyone else had this problem?

Chris Boyes
George Washington University