Is this your machine? If so you can change the Standard Adjustment for how often the extra ball will be awarded. Default is 25% of the time, and it’s based on Rescues. So I think it starts at 6 Rescues when you first start playing the game. That means that if you’re getting 6 rescues or more 25% of the time, then it will stay at 6. That 6 will go up or down depending on how well you play the machine.
You can change the 25% to a higher number and then it will make extra balls easier to get.