First, identify the offending band. Keep in mind that it may not be the same throughout a track or between different vocal parts. Put an EQ on the track and create a BIG (+12 dB or more) and narrow
boost, then move its center frequency around until you've made the sibilance sound as bad as you possibly can. That's where you need to apply compression.
If you attenuate the "S"s but it still sounds piercing, then your filter is too narrow. Or, there is a second band (a harmonic) in play. Also make sure it's actually sibilance you're dealing with; I've seen resonances (from recording too close to a window or an internal resonance within the microphone) cause essiness that masquerades as sibilance but cannot be treated with de-essers. A spectrum analyzer or (better) a sonogram can show you if it's one or the other.
What are you using? A de-esser, a filtered-sideband compressor, a dynamic equalizer, or a multiband compressor? Or maybe a more old-school method?
(BTW, I almost never use de-essers. Unless the recording is really bad, I prefer to address sibilance with hand-planted gain automation.)