Center Channel Extractor sounds different from preview once applied

I am using the Center Channel Extractor to remove a few swear words from a .wav music track for a rather convervative client. This worked well for the first 2-3 instances: tweaking the options in the open CCE window until it sounds acceptable in the preview, hit Apply... all great!

But at some point this stopped working. As before I'm selecting the waveform interval and adjusting the settings until the audio runs smooth in the CCE preview, but when I hit Apply and replay that part of the track, the removed vocals are still there. It might be closer to 70 or 80%, as I can see some slight changes in the selected waveform interval when hitting apply, but to the ear not much has changed.

I know that vocal removal is a fiddly task, and not always, but the difficult part should be in getting the setting of the CCE right. Once this sounds OK, I would expect the applied effect to remain true to its preview. What am I missing here?

I'm fairly new to Audition, but I feel I am not doing anything complicated here: droping the .wav file into Audition, applying a few effects (not using the rack or anything else) and save. So I can't figure out what I might be doing wrong. Any help or suggestions nuch appreciated.

(I tried this on Mac and PC, and the result is the same).