Glad you're aboard.
The low pressure pump/transfer pump runs after the initial start up. It supplies pressure to the common rail in the fuel injection pump which supplies the high pressure to the injection nozzles. Your problem could be the pump has started to fail. Could also be some type of internal obstruction in the fuel supply line. You could remove the fuel supply line and use low air pressure air to blow out the line back into the tank and the try it again. Also adapting a manual gauge to the fuel supply line to the injection pump would give you a reading that if below 20 PSI would indicate the transfer pump is failing.
Best of luck on a solution. Enjoy your adventures and be safe.