You've confirmed that the bonus unit is working properly? You can manually rotate the bonus unit and it rotates smoothly and clicks into each position? And when the ball drains, you can physically see the bonus unit working correctly?

Some items to check:

On Add "Bonus" Unit switch - yellow/red wire and slate/red wire - Is the switch clean and adjusted properly? Any damage to the switch contact?

Z relay coil - Is the coil damaged or shorting out? Does the coil look burnt or overheated? Using a multimeter, you can do an ohms test on the coil.

(Interesting to note that the Z coil is powered by 6 volts on this machine...)

https://flippers.com/coil-resistance.html