I cant gain LP even though i win.

Hello! There is something wrong with my MMR lately. I have recently been trying to to focus on rankeds to get to diamond by the end of this season. I have won about 70% of my last 20 games and hit plat 1. I have just ended my last game and there is an issue. I am only getting 16-17 LP for win and loose 21 lp per loss. What is going on? Im playing with people that are high plat or diamond. Why is my MMR getting lower as i am climbint up? Will this stop eventually or will i have to have 60% winrate from now on to even climb ever so slightly? Yesterday i was getting standard 21 LP per win. What heppened!? Please help. :(