What is the problem with a signal flasher on a western star semi when it stops flashing when the night lights are on?

The first question I would ask is if you were running incandescent or LED lights. If you're running LEDs, and didn't install resistors - or the resistors aren't functioning properly, there's your problem. LEDs take a lot less energy to light up, so to dim the marker lights, the market light circuit needs to have additional resistance added in order for those LEDs to not be fully illuminated.. once they are, they can't get any brighter.