Sounds like a flaky switch. Could be a few things:
1. Switch itself is just unreliable and needs to be replaced
2. Diode on the switch is unreliable and needs to be replaced
3. Some wiring connecting to the switch is flaky and you need to reflow solder there
4. The wiring or a connector is flaky, a visual inspection usually can rule this out, sometimes the IDC connections need to be pushed back in.
Take the glass off and manually test the switch with your finger, see it working in the switch matrix, then start a new game and push the switch. It should make the sound effect for a drained ball.
If you can get the switch to trigger the ball drain sound effect, then drain another and try to see if Million Dollar shot will trigger on ball three.
I've had switches that can be made to work manually, but after sitting a while the switch itself stops clicking. My suspicion is it might be a similar issue and the solution is to replace the switch.