I would start out by removing the burner tube and make sure that spiders or any other bug didn't make a nest in there. If there's a nest, it would disrupt the flow of gas and the heater will not light.
While you have the tube out, check the gap of the igniter. Someone can chime in with that dimension.
Then unplug all the plugs at the circuit board and clean all the contacts. A pencil eraser works good for that. The contacts which can't be cleaned, just plug/unplug it several times to clean the contacts.