[billybek]

Shut off power and gas.

Use a toothpick, nothing metal that might change the size or shape of the orifice.

Try doing that in place.

After try to blow the orifices (I didn't just type that, did I?) with a computer duster. Something that you can't pressure up the manifold with and potentially damage the valve.

You might have a rust or even dead bugs inside the manifold. Try the above first and give it a try. You might need to disassemble further it if that doesn't work.

Edit:
I missed the part where he had mentioned that it would light if he provided some air movement to encourage it.

Blow down the burner tubes with compressed air.
Tap on the burners gently with the back of a screwdriver when you do this.

Try firing it again.