I would look at the ball eject solenoid to make sure there aren’t any loose wires. Then track back from there to find out why that eject solenoid isn’t being fired.

I took a look at a video of a working Twin Win and found that the 'left and right bonus'/'ball kickouts' work independently. There should be two stepper units, one for the left bonus and one for the right bonus. There should also be two relays, one for the left bonus and one for the right bonus. If you don't find any loose wires on the ball kick-out solenoid, then find the relay for the left hole. One of those switches might be out of adjustment. Also, there should be two switches on the score motor switch stacks for the left and right kick-out solenoid as MrBally mentioned. Edit: There is only one switch on the score motor for both eject solenoids. I'll try to find which switches to check.

The schematic shows only one switch on the score motor for both solenoids and they are called the green hole eject solenoid and the yellow hole eject solenoid. That switch is 8A. You can find the two solenoids at F-38 and F39 on the schematic.

What happens when you put the ball in the green hole (right hole)? Does the green kick-out solenoid fire and the ball is ejected? If not, then switch 8A on the score motor needs to be cleaned and adjusted AND/OR there is a loose wire(s) to that switch. If it does, then a switch on the yellow relay is your problem AND/OR the yellow kick-out solenoid is not getting power either from a loose wire somewhere (Jones Plug, the solenoid itself or somewhere else in the circuit) or a dirty Jones Plug. There may be something else I have missed. Please let me know if none of this brings you joy.