You are trying to teach your dog to catch the ball as it is triggered from the box.
I would suggest breaking down the behaviour to reward your dog for catching the ball on the full. (This is without the flyball box)
Start with an easy throw near your dogs mouth, use a verbal cue "Catch" then reward and big praise everytime they catch ball.
Next start throwing the ball with a little more speed from about the knee height which is similar position to the flyball hole and again reward & big praises everytime they catch on the full. Everytime they are successfull increase the speed of the throw.
Introduce the flyball box once your dog is catching on the full sucessfully. Most styles of flyball boxes you can slow down the speed of the trigger by using sponges, this will assist your dog to catch the ball. Once your dog is catching you can gradually remove the number of sponges as it become familar with the speed of the ball from the box.
Send your dog to turn on the box only about 10 feet away (Looks like your dog has a great high turn on the box) and give a second cue to catch, only reward & big praises when they catch the ball on the full. (Don't expect them to do it on the first go, it takes time for them to get co-ordinated).
TIP: Some handlers whose dogs have problems catching, use their dinner to practice catching their food.
Let us know how you go...
Budwiserbear