PSA: If you are an ADC, it is your responsibility to win the game.
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1750085315/33573269?tab=overview
^ My last ADC game. Literally everything that could go wrong did go wrong. Everyone fed, My Support went ADC and tried to take my CS (still shared with her cause I'm nice), Team raging at each other, actively sabotaging each other, etc. I mean, Yasuo had 8 kills and 2 towers by ~15 minutes, and he was just getting warmed up.
I, using Quinn in a role I do not regularly play, managed to fit all this in my backpack, focused objectives, took things whenever I could, and managed to turn the game around for a win. I got myself fed and farmed, and steamrolled through the enemy base by myself, taking special care of the minimap and wards, keeping track of the enemy team as best I could, and generally playing solo.
I used to beleive that there were some games that are just not winnable, but if someone who hardly ADC's at all except for once out of a blue moon, can take Quinn, a highly buggy and "underpowered" Champion, and carry this game, then I don't think that ADC's have the excuse anymore that they "can't win every game." If the odds are 5v5, or otherwise even, and you are the class designed to take objectives, then you can carry that game.
Full Disclaimer: This isn't counting AFKs, DC's, or hardware problems truly outside of your control. This is about those even matches, where you have control over the situation.