Why isn't my piano playing fluent?

Just keep practicing the places you make mistakes over and over again so they become part of your "muscle memory" (which is the memory in your brain that locks in how it feels to play, where to stretch your fingers, etc). Seriously, practice makes perfect. You may think you know it, but there's always room for improvement. Maybe playing it slowly once through will really help you lock in those notes. Maybe practicing the notes in a syncopated rhythm will help. Maybe changing up the dynamics.. once you change things around a couple times and return to the piece playing it how it's written, things should flow more smoothly =) hope this helps!