You might need to have your gas lines cleaned. If there is a blockage of the gas line, you might not be getting enough fuel to start the heater. If that doesn't solve the problem, try consulting the owners manual for these pool heaters. The troubleshooting guide might help you narrow down the problem.

http://www.backyardcitypools.com/images/Hayward/H100-Manual.pdf

Replace the pilot generator if output is below 400mV.