Make sure your solenoid expander board has a 555 bulb in the socket near it.

There's a small square circuit board under the playfeld that has a bulb socket next to it. That's the solenoid expander board, there needs to be a bulb in that socket for the relay to switch reliably. The way it works is it switches the voltage from one set of coils to a different set, which is why your #1 drop is falling instead of the ball ejecting - the solenoid expander board isn't switching properly. Often people either remove the bulb or change it to an led, it needs the full current draw of a 555.

http://www.pinwiki.com/wiki/index.php?title=Bally/Stern#Solenoid_Expander_Board_.28AS-2518-66.29

It's there for the MOC3011 chip, it needs a certain amount of current to switch over. Any LED that's inefficient would probably work, or an array of many LEDs. All depends on the current draw, and the individual components. Some will be more or less sensitive.

Newer designs of the solenoid expander board don't need the lamp at all.