Check the voltages at the step-up connector with your multimeter: verify that your power taps are good.

Where are you getting ground? Check your ground continuity to the ground braid to make sure it's good. I just tap the ground braid directly.

You can then verify the circuit is working by unplugging from the Q24 harness connector and briefly touch that P1 signal wire to ground braid. Knocker should go off.

How are you testing that it's not working in the game? Have you played to a point when it should knock? Have you run the knocker test instead of just the coil diagnostic test?

I just noticed on my ST Premium that my knocker also won't activate in the coil test, but I know it works. There's a separate knocker test in its own menu option, that one works for me.

Knocker test isn't hard to find: just scroll to the right of sw, coil, flash, lamp options: right before and next to the speaker test.