If you look at my game history you’ll see that I have won a lot of games lately, however lol ,
I’ve been stuck at Platinum six it will not go up to Diamond no matter how many games in a row and win… 
Winning isn’t the sole method of ranking up, you also need to prove to the system that you are playing better and deserve to go up in rank. This prevent issues in previous games where lower level players could get boosted to ranks they don’t deserve. For a more detailed explanation of this, please see the matchmaking feedback topics that are pinned at the top of the matchmaking forums. The current June topic along with the previous 2 provide extensive commentary from Dr. Menke and other community members about how the system works