This should be an easy fix.
Every aircraft has a AIRCRAFT.CFG file and it is this file that controls things like Fuel the below is an example of a PAYWARE PMDG 737-800
[GeneralEngineData]
engine_type = 1
Engine.0 = -5.943, -16.470, -2.880
Engine.1 = -5.943, 16.470, -2.880
fuel_flow_scalar=1.2105min_throttle_limit = -0.3719
max_contrail_temperature = -30
[TurbineEngineData]
fuel_flow_gain=0.002
inlet_area=22.619
rated_N2_rpm=13200.000
static_thrust=24200.000 /
afterburner_available=0
reverser_available=1
AfterBurnThrustSpecificFuelConsumption = 0
[fuel]
Center1= -1.884, 0.000, -1.800, 4299.000, 0.000
LeftMain= -9.110, -16.500, 1.600, 1288.000, 0.000
RightMain=-9.110, 16.500, 1.600, 1288.000, 0.000
fuel_type=2.000000
number_of_tank_selectors=1
electric_pump = 1
fuel_dump_rate = 0.0
These are the sections you need to look at
and the order they should be in
the line that is in bold is what determines fuel flow
most of the numbers you see here are where the engines and fuel tanks are positioned
the last numbers are how much fuel is in each tank
in this case
center 4299
leftmain 1288
rightmain 1288