The trough that the balls set in has switches that determine
how many balls are in it.
At the beginning of a game there should be 8 balls.
You could look at the switches under the balls in the trough.
I also found a couple of times, on my Bikini machine, that a ball got
caught up on the "return road" back the the trough: so the machine's
trough thought there were not eight balls to allow a game to start.
This may help,
Terry K