I have no clue if the pressure changes when the heater tries to fire. I do not have tools to test that.What is the gas pressure before you start the heater?
Does the pressure change when the heater tries to fire?
What is the exact model number of the heater? (Part number p/n from the sticker).
Maybe remove one of the bottom manifold bolts to see if water comes out.
Check the voltage and current going to the igniter when the heater tries to fire.
Maybe some pictures would be helpful.