On the back wall of the cabinet is a circuit board with power resistors for the flash lamps. I would check this first. Look for shorts.
On my machine, one of the power transistors on the cpu board was blown. So someone just cut out the power resistors to prevent the flashers from operating. Only thing was that they cut out more resistors than the one with the problem. So several flashers weren't working. I installed new power resistors then found which transistor was blown, then replaced it.