A good fly cast starts with the grip.
If you have ever received lessons in golf, most swing flaws can be traced to your hands and how you hold the club. This holds true with your fly cast.
You need to keep a firm grip. The line goes where the rod tip goes. (Burn this into your brain).
Because of this, hold your thumb on top of the grip, and snap your casts, to a firm stop.
When you stop the rod, visualize looking “through” your casting thumbnail, odds are that the line will unfurl right through that window.