The boy looking for the ball will appear four times. Pay close attention. There will be more guests halfway through the game. Maybe you missed it and took that boy as a guest.

I tried many times again, and finally I know why you are like this. You click on the same tennis ball many times in a row. At present, the ball has been scored, but the boy looking for the ball is stuck and no longer appears. This may be a bug in the game. Try clicking the ball once, and it will appear four times.