In Stern games, turning on bill validator just flashes the the picture on the screen.
You need to get the switches correct on the bill validator. And if you don't know, turn game on and off and try different settings.
The ones I would start with is if it has a Gaming or Vending switch, try Gaming. Then Harness Enable, or Always Enabled. Try Always Enabled.
Once working, then set pulses per dollar, I think 4.
The other switches are usually bill direction, high security or no, and Ones,Fives,Tens,Twentys.
LTG : )