The weaker your opponents the easier it is to do well.
If you have been getting the S rank regularly, then is it safe to say you've been winning a lot? If so, then your getting matched with ever better opponents of course, and thus it gets ever harder to get a better score or even maintain the same score.
But that's all just my guess. Nobody really knows how scoring works and Riot's never gonna tell, so we can only guess.
Perhaps switching game modes will help considering each one has its own MMR, but I've heard not all game modes give out those mastery tokens (at least ARAM doesn't) but you can try ranked and normals. If you don't normally play one of those then your MMR on that mode should put you with weaker opponents that you should be able to stomp for a high mastery score. At least that my guess again.
Good luck.