First time I’m playing this machine (Dragon’s Treasure) and I wasn’t understanding how one was supposed to get the bonus. I went to the machine to the left of this one and understood immediately…
As you can see in the picture, all gems are staying lit up on this machine, therefore the bonus is bugged. No matter where I drop the coins, the gems stay like that, the bonus never works.
Works just fine on the machine next to it though: