Data East Jurassic Park (Data East, 1993) problem

[kingpin]

i very recently bought a jurassic park pinball machine (data east):D and it is a lot of fun , only one problem so far that i can't solve , when it starts a game it loads two balls at once then struggles to shoot them out , if i take one of the 6 balls out,the problem goes away and the game works perfectly but the machine then gives the ball missing error message a few times between games. what could be the reason for this prob ?