I found this description about Last Chance Feature, it mentioned that this only works with the last ball, maybe that explains what you see?

LAST CHANCE FEATURE - A captive ball will become a ball-in-play when the player's last ball enters the outhole through a side outlane. The last chance feature does not occur if an extra ball has been won. If 1 ball is captive, it will be released for play when the player's ball exits either outlane. If 2 balls are captive, only 1 ball will be released, corresponding to the outlane the player's ball exits. The LAST CHANCE feature can be disabled with game adjustment switch #31 in the off position.