You have 5 losses on Tahm Kench. As a general rule, don't play new or recently reworked champions in ranked until you have thoroughly practiced them and they've become established in the meta. (So, maybe you did practice him well enough, but your teammates may still not be familiar with how he works).
I haven't been past Plat 4, so I can't offer much to get you up into Diamond. Though, I've noticed while being in Plat that players know how to play against Blitz really well. Since he's your second-most played champion, you may want to focus on playing Thresh or another support more. But, if you have fun playing Blitz, then by all means do what you have fun with!
Good luck! :)