Mastery Tokens

Hey guys so I've played many Lux games ever since the new patch for the new tokens 6 and 7, I'm currently level 6 on Katarina. Lux and jinx and after all those games I have earned a s+ or s but I don't get a mastery token? Is there something I'm doing wrong because i really don't understand what I have to be doing to receive the token?? plz help <3