Why do i win most ranked matches but lose all promotional games?

And most of the time it is due to teammates either feeding afk or just basically not giving a shit.

Is there a role or champ that will carry 1-2 dead weight players? I have looked and tried to fix my mistakes as well, and i have become a better player as shown in non promotional games. But how do you fix this? this is abut the 7 or 8 time i have failed the series due to teammates. It is like i am stuck in a never ending loop. Lose promo, win 2-3 in a row, lose promo, win 2-3 in a row, etc. (not every time is it 2-3 in a row but it is very close to that pattern).