What is wrong with Ranks and Matchmaking?

I have a few accounts that I'm trying to climb on. One of them was placed in Plat 3 and I play fine against Plat 3-Diamond 5 levels players. However, on my other account I'm currently stuck at Plat 5... no matter what I do I cannot win. I was placed in Silver and made my way back to Plat with ease except this time I've been stuck in Plat 5 and I'm winning about 25% of my games. I will win lane every time but end up losing. I don't know what is going on? I main adc/sup. I get queued with people from Plat all the way to silvers and it's all random. How can I play consistently with high platinum and low diamonds but cannot carry out of Plat 5? Does anyone have this issue or is it just me? Why does it feel like Plat 5 is different than all the other ranks?