If you look at index 11E on your schematic you'll find relay"Q" the bonus score control relay. Drop down to 12E where the Add "Bonus" Unit coil is, if you follow it's path to the right, it's activated by Q and a motor switch at 1A. Now looking to the upper left side of the schematic on the 6 volt side at index 5 D is relay "Z", energized by a switch on the Add "Bonus" Unit.
Check the bonus unit switch to Z again, with the slate-red and yellow-red wires, make sure it opens after the add bonus stroke.
Also note that bonus isn't collected until the ball drains...then the bonus unit steps 15 times and the "15th position switch" closes the path to the ball return relay"O".

The switch at the bottom of the bonus unit that closes on each step makes the connection. When the ball rolls over a number on the playfield ( or hits a target ) the corresponding relay in the bank under the top of the playfield trips. That sets up the path that the switch completes. The switch only closes for a instant at the end of each stroke, as the bonus disc shoe touches each contact. If you start a game with the playfield up and trip some of the rollovers and targets, you can manually step the bonus unit and see if you're getting contact to the Z relay.