Applying Ambient Noise Not Working

So I'm just learning Soundtrack Pro and I learned how to set an ambient print and apply it to replace noise. First several tries were "Whoa! This is way cool!" as I'm working to clean up a badly recorded project. I would make a change, switch back to FCP, and squirm with pleasure as I heard the much improved change. Now, however, half-way through, it's stopped working and I have no idea why. I make the changes in Soundtrack; it works; but when I switch back to FCP all the unwanted noise is still there. What happened?