I was stuck at 15k trophies for weeks and couldn't figure out why.
Kept blaming the matchmaking. Kept thinking one card was the problem.
When I actually ran my own battle log through the AI the first time it told me I was playing a Cycle deck like a Beatdown player. Waiting to build big pushes instead of just cycling fast.
Obvious in hindsight. Completely invisible when you're in the middle of a losing streak.