Why doesn't my Trane rooftop furnace ignite?
CHECK FOR GAS AND THAT IT IS GOING IN THE RIGHT PLACE

I would check the gas pressure and the orifices or venturi. If a bug got in the orifice closest to the igniter gas will flow to the other burners but may not ignite. A bug in the venturi can disrupt the flow and cause a failure to ignite. Low inlet or outlet gas pressure could have the same effect.

This is in respect to a barbeque but the premise is the same
www.barbequeworld.com

This is where the orifices attach
www.repairclinic.com 

And this is what the burner should look likewww.repairclinic.com