Hi Craig and welcome to the site.
Probably first thing to check is that all the balls have returned down to the front of the machine under the apron (3 in total), then check the switches the balls touch to make sure they are working. Otherwise if a ball is missing from there, common place with the white water (mine any way) is that it gets stuck in the dimple formed where the ball shoots up to the top playfield via the wire ramp feed or possibly somewhere else in the game.
Hope that helps as a starting point if you haven't already check those things.
Enjoy it is a awesome game especially with the latest chip.
