Hello
This must be the detection of the outhole that doesn't work.
At the game start, for the first ball, its presence in the hole is not controlled and is always ejected. But for the second ball, the CPU look for its return in the outhole.
Check the outhole switch and also the J7 connector.
On SYS 1, this switch is wired on the J7 connector, such as the others switches of the matrix - but is not part of the matrix.
Using the test procedure, it's not possible to check if this switch works or not : the test 13 detect all closed switches on matrix, but not the outhole.
You can easily check if this switch is detected or not: add some bonus (drop targets). Then when the ball is loose, if the outhole is detected, the bonus should be decrease. If not, the switch is not detected.