S+ to D? can we get more info on how these happen?

As a very active player i always want to know how I did, and the ranking system at the end game lobby gives you a decent score on what you did. Although when i get a B or A instead of an S- like i believed i was doing, the game is reluctant to tell me how or why i got a lower score than my teammate who seemed to do worse than me overall. At the end game lobby....maybe have a graph on what you did that increases this ranking at the end game lobby or something that gives you more information on what you can do to get that S- instead of just relying on hope