The way your kickers work is:
- the two switches on either side of the kicker directly fire the kicker solenoid between them (last two coils at the bottom of the schematic).
- under each of the two kicker arms is a switch that should close when the kicker fires, which should in turn fire the 10 point relay (18-19 C on your schematic).
- the 10 point relay does all the heavy lifting (points, chimes, match unit, etc.) as shown in the manual:
Wild Card 10 pt relay (resized).jpg
So you need to figure out which of these isn't working. Start with the four kicking rubber switches and make sure that all four reliably fire the kickers. Then check that the kickers always fire the 10 point relay. Narrowing down where the problem is will identify which switches aren't working.

Your problems could be as simple as dirty contacts but you'll likely need something with a little more oomph than just alcohol and a swab. What you need to remove likely will require a mild mechanical abrasion. Start with a dollar bill or business card. If that doesn't work, try a flexstone or very fine (1000+) sand paper. Use the least aggressive thing that will do the job.

/Mark

How should the bonus work? Does it award 5000 points at a time, then take one step down, then award another 5000 points until it reaches zero? And how does your bonus work? For example if you're at a 15000 point bonus and the ball drains, do you get no bonus, or do you get nothing until it steps down to 10000?

There are a few interesting switches on the X relay as you suggest. I would carefully examine that relay and activate it manually with the power off. Do each of the switches open and close as you move the armature back and forth? Do all of the contacts on the longer blades over travel some and push back (not just touch) the contacts on the shorter blades? Also consider cleaning all the switch contacts on the X relay carefully.

Same goes for your score motor switch stack. With the power off slowly turn the cams and watch the switches move up and down. Each switch should open and close as the cam turns below it.

It looks like you are missing a couple of switch blade lifters. The part numbers depend on the length of lifter:
Switch blade lifters (resized).jpg

I found a video that shows that the bonus first counts 5000 points as five individual 1000 point pulses if appropriate, then counts the rest of the bonus as individual 10,000 point pulses. Here's how that works:
Wild Card 5000 point bonus (resized).jpg
As the ball is played, the Bonus Unit takes one step up for each 5000 point bonus earned which advances the red arrow in the schematic through the various bonus positions. If the ball drains and the Bonus Unit is not in its zero position, the Bonus relay fires which closes the switch in the upper right corner. If the Bonus Unit is in the 10k, 20k, 30k, 40k or 50k position, the X relay in the upper red box immediately fires and the bonus gets added to the score 10,000 points at a time until the Bonus Unit steps down to zero.

If the Bonus Unit is in a position that requires 5000 points (5k, 15k, 25k, etc.) the X relay does not immediately fire when the Bonus relay switch closes as it does above. Instead, the X relay stays inactive while five pulses from the score motor impulse cam are sent to the 1000 point relay through the circuit in the bottom half. Once 5000 points are added to the score, the Bonus Unit steps down one step (to 10k, 20k, 30k, etc.), and then the X relay fires which changes the position of the Make/Break switch in the red box in the bottom circuit and prevents any more pulses from getting to the 1000 point relay. From there the remaining bonus points are added through the 10,000 point relay.

So I'd see if your X relay holds off for a score motor cycle (a 180 degree turn) before firing on a 15k point bonus. If it does, then check the switches in the bottom circuit to see which one isn't closing properly.