Spitballing here, no experience with these pumps.
I have a Honda 2200w generator, hooked to a boat tank with a standard boat connector, and an adapter on the generator fuel cap. The generator has a fuel pump, so while the normal fuel cap just has a vent, the adapter is sealed and makes it draw fuel from the remote tank. Works fine for the most part, but I've had the fuel connector not open the internal valve when connected, not pass fuel, and choke out the generator when it pulled a vacuum in the generator fuel tank.
Maybe something similar is happening to your fire pump. Remove the fuel connector from the lines, connect the lines with a clear fuel filter so you can see movement, and see what it does. If that fixes it, replace the fuel connector.
I have a Honda 2200w generator, hooked to a boat tank with a standard boat connector, and an adapter on the generator fuel cap. The generator has a fuel pump, so while the normal fuel cap just has a vent, the adapter is sealed and makes it draw fuel from the remote tank. Works fine for the most part, but I've had the fuel connector not open the internal valve when connected, not pass fuel, and choke out the generator when it pulled a vacuum in the generator fuel tank.
Maybe something similar is happening to your fire pump. Remove the fuel connector from the lines, connect the lines with a clear fuel filter so you can see movement, and see what it does. If that fixes it, replace the fuel connector.