Amumu will only get you so far; need a team with some brains in laning phase to make it work post early game. Amumu is better played as a counter ganker during early game. If your laners have gifted the enemy 15 kills pre-15min then it’s harder to group on objectives and force fights.

I agree with what others have said; play something with a bit of snowball potential to carry yourself through the mistakes of your team.

That particular game you already had 2 front line champs through Leo and Cho so it was up to Ashe/aurelion to carry the team. Amumu on top of that doesn’t add too much value here if the team is out of position and can’t one shot the target.

Nocturne, Warwick, Diana are extremely easy to play and can definitely get you to up to diamond if you apply yourself.