If no spark is visible check connections of lead wire at piezo ingitor and ceramic electrode. Pilot orifice is clogged. Cleaning pilot orifice Excess gas pressure. The gas valve on the heater has an internal appliance regulator within it. This internal regulator will lock up if excess gas pressure is delivered to it, shut off gas supply to unlock internal regulator.
Pilot flame is small or yellow.
The pilot flame can be viewed through the lower right view window while the control knob is depressed. If the flame is small or yellow in color then the pilot orifice should be cleaned. If pilot flame is adequate then the thermocouple should be replaced. If room is already warm dial may need to be turned almost all the way. Main burner orifice is clogged.