There are three switches on the standard Sys1 slingshot arrangement. The two above the playfield will fire the coil. The third is in line with an actuator arm attached to the coil, and tells the MPU that it needs to score and make the sound. When the coil fires, it "kicks" the slingshot AND trips that switch. That third switch should be visible under the playfield, between the two switches sticking up behind the slingshot. Odds are that switch is misadjusted, corroded, snapped off, or has become disconnected from the system.