Rating system

So, today after a short break of league, I decided to play a game. As usual i pick my favourite Udyr. We are losing the game since 1:00 to 20:00. I have had always positive score and working my ass off trying to carry my team. I basically played 1v5. I ended with score 16/6/12. We had a zed who was just "securing kills" and went 19/6. I dont even know how he did more damage than me. Anyway i was playing off-tank udyr with ap build. After carrying my team to the victory (thanks varus, you helped too) the game gave me an A. I need a mastery rank 7 token to up my Udyr to level 7. I have been getting good scores and doing everything possible to get it, but it seems that i cant. Once i even went 23/5 with udyr offtank on top and i got A+... Can someone tell me what im missing?