[Solved] SONAR Production Tips – Building A ProChannel De-esser

Try feeding the side-chain of the compressor from a completely different track to get a grip on it and make sure you're using it right.
 
To de-ess, you want to route a signal that has radically exaggerated sibilance to the side-chain of the compressor (which is processing the main track), so that the compressor will react primarily to the sibilance.  The main track will then be compressed at these points, making the esses quieter. 
 
If you're going to take the trouble to create your own de-esser, though, it may be worthwhile to use a method which mixes phase-inverted audio of the esses back into the audio, since this is an effective method of de-essing which is not commonly available in plugin form (I think Cubase's standard de-esser does it, but I can't think of another).  If you wanted to automatically do it this way, you could perhaps use the Sonitus gate on a duplicate track.  Set the Sonitus gate's filters to the range of the sibilance, then flip the polarity of the duplicate track.  The result should be that only the esses pass through the gate, and then are mixed back (phase-inverted) into the audio, cancelling out the ess sound from the main track.  Lower the level of the duplicate track to lower the amount of de-essing.
 
In reality, with modern audio software, it doesn't take much time to simply automate volume or a gain plugin on the vocal track to lower the esses, and it tends to be more effective than trying to set up even a dedicated de-esser to work on all the different esses.