Propane back up-not

I have a munchkin condensing boiler that won't fire, the code says the pressure is low. The wood boiler runs the P/S at 20 psi, and when it's hot the propane runs up to 15psi, but as the wood burns out and the P/S cools to less than 100* (which is where the propane boiler aqua stat is set) it won't fire and the pressure at the prop boiler drops to zero?

I can't find any leaks in the system and even closed the air bleeder valve with no luck

i put a gallon of water into the system and this brought the pressure up to 24psi and the boiler fired