Are you working in BiaB or RB?
I'd go back to the specific vocal track and work on the EQ for that first.
Get that so the 'S's are nice and clear. Then remix it.

Sometimes EQ works great, sometimes the frequency isn't there enough in the original track, at which point you can try some enhancement type tools that do math and add related harmonic frequencies to the track, but you have to be careful because it is easy to abuse, and easy to focus in on the example given 'S' and not notice how else it is affecting things.
To combat this, I usually take a break and then listen back to the result in the mix with fresher ears.

/just a safeguard against falling into the previously mentioned trap
//But do try simple EQ first, find the missing 'S' frequency and add more to start .. if it works, that is usually the quickest way
/// sometimes parametric EQs (that allow you to home in on the desired frequency) work better than graphic EQs, depending on your abilities with both

I do not work here, but the benefits are still awesome
Make your sound your own!