champion mastery shards, what gives riot ?

hey so just last night i was playing in a whole bunch of matches with kha zix jungle and doing really well with it, there is one match in particular i would like to draw your attention towards. i was in the jungle with kha zix and made a whole bunch of successful ganks during the early part of the game helping my team mate to a victorious lane, had complete control of either jungle, had a fairly decent amount of farm for a jungler, full control over dragons ended game with 4, got rift Harold and baron, ended the game with 20 kills and 2 deaths and yet i was not awarded with an S or S- for this match, this leaves me wondering what in the world does one have to do to get these.