The way to do it is:
Watch the point with some head movement. You can't just focus on the line nonstop. If you want to understand why, just try staring at one precise spot for several seconds, and it can get blurry, eyes get tired, etc.
When the player hits the ball and you know it will be threatening your line, move your eyes to the line where you think it will land. The key is to get your eyes to the line before the ball. That way, your hea won't be moving (which is why parents, players, spectators, coaches). If your head is moving, you will probably miss it. I you get there late, you will probably see it as the ball is coming up instead of landing.
Use some head movement (not just eye movement)
Go to line when your line is threatened
Get to the line before the ball
See the ball
Call the ball in/out.