Records with words with the letter S are slurred

The problem is usually a combination of VTA and VTF. Try lowering the tail of the cartridge slightly and also increase your tracking force a little. Should help immensely.

Shakey

Some singers "SIS" the Ss, with "natural sibilance" in their vocals.

If all is adjusted very well, it is often a cartridge problem. Some boost this frequency range. I had same prolems with my Lyra´s, now with an EMT the sibilants play very well.

+1 to Lewm's and Zd542's posts.

Everyone else is just guessing. One or more of these guesses may be correct for your case, or not.

As I previously posted and as these responses make clear, there are many possible causes. Which one(s) apply to your situation is unknown and unknowable without additional information.

In my experience what you are hearing is an artifact of mist racking. Some records are pressed "hot" and sibilance is unavoidable. Goodxalignment and set up are key and having a cartridge that tracks well also helps