Problem with my Mendota Seabrook Model D-30 Direct Vent Gas Insert
A non-working pilot light that doesn't remain lit could have blockage in it's line, but most likely at it's orifice where the gas pathway is narrowest.
If the main burner ignites and burns,but than shuts-off than it could be the thermo-couple needs replacing. First make sure the top 1/3rd of the thermo-couple end is positioned directly in the pilot flame otherwise it wont send the electric signal that is generated by heat to the gas valve to remain open.
I'm not sure all heaters have this saftey feature, but if your exhaust vent draw is not adequate (that is there is blockage) the heater may be sensing this and as a precaution shutting off the gas flow.
Lastly, and i find this to be a common problem, check/replace the thermostat. If it's a basic (2) wire model remove it's face to expose the wires. Should be (but not always) a Red and White. You can jump these by bridging them with a similar sized wire. The heater should ignite (make sure the pilot is working first).