First arrow may work at that bowling center, but as season and temps change, you may lose that shot. A good coach that can teach you to read what the ball is doing, and how to adjust is well worth the price. Secondly, spare practice. You should be a legit 95% on your single pin spares. Many people claim they are, but in reality, they are 80% and that drags their average down 15-20 pins per game.
Finally, back to the coach, simple mechanique flaws can drastically affect you game. Even if you have perfect mechanics, and you start to struggle, bad habits can creep in and kill you scores without you realizing what is going on, a coach will spot these issues.