Mastery Points and tokens

Hello! :)

Ok, Ive ran out of ideas for this one. So lets hope that someone here as the answer. I havent got a token for S- or higher in ages. Im lvl 6 Nami, and Ive been getting S a lot, or S+, but token never showed up. Is there a reason for that? I have 2 tokens from previous season, and it seems I cant get the last one so I can go lvl 7 with Nami (or Soraka) .