How do you get mastery tokens?

I've been playing blitzcrank consistently for 2 seasons. I'm still at mastery 6 with 2 (of the 3 tokens) needed to get rank 7. This week alone of the 5 blitz games I've played in Ranked I've gotten an s and s-. Not saying that i normally get a high percent of "s"'s but I generally have gotten 1 to 2 S class's a week. The last time I got a token was about a year and a half ago? What do you have to do to get the last mastery token? Am i just really unlucky or is my account bugged? Last season i had 107 games played with him, and 15 this season. Any information about getting the mastery tokens in general would be appreciated.