I have some external fuel tanks connected to a fuel line adapter part, that's connected to a strut part, that has fuel line enabled. Even with fuel tanks filled with the correct fuel, connected, and fuel line enabled, the engine at the bottom connected to a fuel adapter, that's connected to the strut part with the fuel tanks, doesnt receive fuel from the tanks.
I switched out the strut part out for a fuel tank part of same dimmensions, using same correct fuel. This time the external fuel tanks were able to connect fuel lines throught the center fuel tank to feed the engine.