I have a 15 yr old Teledyne Laars 250K BTU heater. It has always taken 5-6 times to lite off initally since it was new. Last fall I ran the tank completely empty. I had the tank filled this summer but can't get the heater to ignite. I can hear the ignitor clicking (presumably sparking) but the sound is not as loud as it was in years past. Only thing different this year was the addition of SWG. Any suggestions???