The bell is adjustable. If you unscrew that screw on the top you can slide it closer to the striker arm and see if that helps your issue, if it is stepping properly as Lee described above.

I had a game where I attached washers to the end of the striker arm. I never did like that design. I always felt it was a poor one. And in any case, if you're comparing it to the sound produced by bells using a solenoid/plunger combo, it will never sound quite as good.

Glad that helped. Sometimes on here we're all guilty of overthinking problems.

It doesn't look too dirty, but the game probably has those lousy metal coil sleeves in it. If you can get it out and replace it with a nylon sleeve it will be tons better for you. Doing that and shining up the arm plunger will also make the unit so much better.