Quoted from Grangeomatic:
First, do all of this with the power off. You don’t want to accidentally short some sort of coil voltage into the switch matrix. That’ll give you way more problems.
To clean the contact, take a business card, dip the corner in some rubbing alcohol, then slide that part between the contacts. Use a finger to gently hold those contacts together, then slide the business card out. Repeat a few times.
Then test the switch. You may not need to adjust anything.
Still not working?
Inspect how the contacts act as a ball rolls over the switch. If they’re obviously making contact, don’t mess with it. If not, needle nose pliers are OK if you don’t have one of these
Gently bend the switch so that they’re normally open, but making good contact when the ball rolls over the switch.
Also, inspect the wiring to the switch. Give each wire a gentle tug to make sure it’s still connected well.
That’s how I’d start.
Remember, power off.
Great advice here!
Only thing I would add is that regular copier paper will work to pull through the contacts. I've never used alcohol either so that would be optional IMO. Gently press the contacts together as you pull the paper through. Your goal is to wipe off the area of the contact that touch as the switch is closed by the ball. If you mash them together too hard, you might turn the contacts to an area that never touches. Pull the paper through 3-4 times using various levels of pressure. You will see black streaks on the paper that get lighter with each pass.
NEVER use abrasives (file, sandpaper, etc) on these low power contacts on solid state games. They are plated and sanding them will wear through the plating resulting in a contact that never work for long.
The adjusting tool that Jeff linked above is a godsend for adjusting these types of contacts. Highly recommend picking one up when you next order parts.