The hextech system doenst work at all

i was playing a 5v5 blind pick to get a hextech box becuase i have alot of keys but i never seem to get higher then a A- what is stupid because i always play very good with certain champs i get like 8 kills min per game more if they do not surrender but i do everything to get an s- or higher but i dont seem to get higher then a A- wich i dont understand if i do everything correctly