Thermal Couplers/Thermocouple??

Check your propane pressure first sounds like you may be running out.Second some ignightors act as a flame prooving device where they light the flame and tell the computer that it's verified ignition before opening the gas valve and some older units have ignightors and a standing pilot with thermocouple.I would need more info to really tell you what's going on so post another ? and provide more detail please