Ranking

Hi, I’ve been playing Halo 5 for a while now and I don’t know where to post. I main Arena Swat and I can’t rank up anymore. My Usual rank is High Platinum-low Diamond. I played my placement matches with a friend who is also in low Diamond, but I got placed Platinum 2. I won half the games and went positive on my average KDA. I didn’t know what to do so I just kept playing. but every time I would win, my Rank wouldn’t move. And every time I lost I would lose more than Half the rank. So to see if I had just gotten worse at playing or something, I switched to my alt account. I started it up, played the placement matches, and I was placed Diamond 2. What?! Is my main account just broken or something? If so, Is there a way to reset my rank and MMR? I know it’s late 2019, but any help would be very much appreciated.

-Mike