On gas appliances with power vents, the unit wants proof that the power vent is sucking a vacuum before it lets the main burner come on. Some place on the vent fan housing,there should be a flexible hose of maybe 1/8 to1/4 of an inch. This hose will go to a vacuum switch that allows the main burner to come on. First,check the hose for cracks where it fits over the barb fitting. Then check the barb fitting itself. I have seen them get crudded up with dirt and deposits. A little drill bit turned in your fingers usually will clear them. Spiders love the 'gas smell' and will build little blockages in dormant gas appliances. You can also check the vacuum switch with a continuity tester to see if it works if all else fails. Make sure that the discharge pipe or chimney is not plugged as well
Any time I have had problems,a cleaning solves them with no parts needing replaced. Hope this helps, and please tell us how it goes for you.