By Atwood boiler I'm guessing you're talking about your Atwood water heater? Need much more details to give good help, I'd start by looking for a manual and/or troubleshooting guide for your water heater model.
They require 12v power and LP gas at the proper pressure. A thermocouple that senses the flame and allows gas to continue to flow is a common issue, also the overheat sensor can go bad and prevent proper heating. Some have issues with an insect in the air tube, wrong gap in the ignitor, etc.