Hi Sean,
Does this happen on a single player game or just when you have more than one player (or both)? I'm trying to remember an issue I was having with one a while back, and I think it was because the player unit in the back box was not advancing when I had multiple players loaded.
If the Player Unit is advancing, then take a look at the DX,EX, and FX relays. From what I can tell on the schematic, EX would need to be closed for the ball 2 light to come on (assuming PU is in the correct position)