Do you have another game with a Squawk and Talk board? You could try swapping boards between games.
I am certain Centaur requires 4 balls in the lower trough and one in the outhole. I just got up and checked my game to verity. 5 balls total.
*edit* the captive ball isn't a normal pinball; I believe it's 1/16" bigger, possibly the same as what bingo games use.