Why can't I win a single ranked game?

I've had plenty of times where I went on winning streaks, losing streaks, in between streaks. But now it literally seems like I can't win any ranked games ever. And a common occurrence I notice is that my team, with extreme frequency starts to throw by about 15 or less minutes. I can understand a few bad games, but this has been happening for months to me now, regardless of how I perform. It's actually starting to piss me off because I've climbed way past silver 4 in the past, but now all of a sudden I've been dropped all the way down to 5 and can't win a single game.