Quoted from o-din:but pressing the start button again while the ball is in the shooter lane does not add player two.
Usually the second player can be added up until the point at which the game thinks points have been scored on the first ball. This is usually done via a switch on the 1st Ball relay (U). The U relay should energize at power on, and stay on until the game is reset. Then after reset, while U is still not energized, a normally closed switch on U allows the second player relay to energize and latch when the start button is pressed again. (Usually this is the CX relay.)
But if points are scored before pressing start again, then U energizes again, opening the switch that allows the second player to be added.
So it could be a dirty contact or open switch on U.