Why won't your laars lite pool heater turn on The heat exchanger temperature sensing bulb and siphon loop assembly have all been replaced with no leaks.?

When was the last time the filter was cleaned? Have all the components in the heater system been checked for proper values? Does the heater sound as if it wants to fire up? Does it have a pilot light or electronic ignition? Have the orifices been checked for bugs or Spiders?

Ken