In an e/m game, it's okay to use a small contact file to restore a good connection on the contact points. Electronic games use gold-plated contacts (except for high current uses like flipper circuits), but even on those cleaning the points would still be the first thing to try for an intermittent rollover switch like this. The different response between operating the wireform and the contacts themselves suggests to me that there's something odd there.
Otherwise, you'd need to follow the circuit for the 'Advance Ball Count' solenoid across the drawing, also checking the circuit for any relay-operated switch involved, such as an 'Adv. Ball Count Relay' (if there is one), i.e. the runway switch might operate the ABC solenoid directly, or it might switch a relay, which operates the solenoid. From what I remember of a Gottlieb wedgehead I once owned, the motor isn't used to advance the ball count (not from the runway switch, anyway). If there is a relay, you'd see/hear it working when the contacts do close properly.
In fact, the solenoid can't only be operated by the runway switch; when the last ball drains the solenoid's supposed to operate from the outhole switch to go into Game Over. AFAIR the motor's used to do this. My wedgehead had worn contacts on the ball count units' wiper disc, and it would kick the outhole again, with the runway switch putting it into Game Over.