The S ranking system is bugged

Dear rito ive been trying to get my last and final M7 shard for Ashe. The other 2 shards were fairly easy to get but this one is a pain for some reason. Its taking atleast 20 games to get and i still cant get it. I just had a 40 minute game where I went: 6/1/14 with 248 cs. All I get is A's and i jsut want my M7 ashe shard so please fix it .