Check the switch first.
Next you can try replacing U8.
This is a 7402 chip.
I'll attach some schematics for System 6 driver board.
Look for solenoid 19, driven by Q6.

Also I would recommend putting in a Special Solenoid Saver Board, which fuses each Special Solenoid.
https://nvram.weebly.com/repair--conversion-kits.html

And a Bridge Board which can prevent your game from catching fire when a short occurs in one of the rectifiers in the backbox.
http://www.inkochnito.nl (click on the Bridge Board image).
Available at RTBB https://www.rtbb.com.au/product/williams-data-east-bridge-board/

Peter
http://www.inkochnito.nl

Williams_System-6-7_Driver-Board_layout.pdf

Williams_System-6-7_Driver-Board_schematic.pdf

Are you sure you are looking at the right Special Solenoid?
In the first post you say "Left Bumper", so I assumed it was the Left Pop Bumper (Solenoid 19, Q6).
If you are talking about the Left Slingshot (Solenoid 22, Q12), then you should try and replace U9.
If things still does not work, U7 is next in line.

AlienPoker (resized).jpg

....

You are talking about the Left Slingshot (Solenoid 22, Q12), then you should try and replace U9.
If things still does not work, U7 is next in line.

Check if the slingshot coil is connected correctly.
Red wire on the banded side of the diode....