First thing to check is to make sure there is a plunger in the coil and it's moving freely. Check to see if the plastic white tip is broken off the plunger. Next, see if there is a rubber grommet on the bottom of the bracket that it is resting on.
You don't want the plunger to be sitting on the metal frame directly. Check to see if you are getting power to the coil. Ohm out the coil.

Plunger resting on the rubber grommet & the other end with the white plastic tip.

plunger (resized).jpg

plunger 1 (resized).jpg

I would give it a quick mechanical check.

Then I would jumper the two lugs to the 25v on the transformer. (just touch the leads). If if knocks, you know the problem is electrical.

Then, start jumper all from the RED/White wire past all the NC switches on the Motor and activate the replay relay. If it knocks, the problem is in those switches. If it doesn't the problem is in the replay relay or the feed to it.

Continue moving the jumper until you find the issue. My money is on one of NC switches in the motor, which sucks, because they are hard to work on, especially for a newbie.

Edit: This is how I would do it with jumper wires. In reality, I would likely use a test lamp and see where the defect lies after jumpering it directly to the transformer to test the knocker itself.