Fly casting needs practice.
Like any activity which involves complex body movement, fly casting needs some training.
Being out on the water and fishing is not casting training. It's kind of similar playing an instrument. There is no time during a gig to practice playing guitar. You need to rely on your aquired skills. During fishing there are no ressources available in your brain for thinking about casting mechanics in detail.
As a certified FFI Fly Casting Instructor, I love to teach casting mechanics in private lessons or workshops for groups.