Just bought this myself, but havent experienced problems you are experienceing.
Th 4 main engine tanks will (should) gravity feed anyway.  Is it possible that it is the GTC still running?  The simulation appears to have a tiny centre tank? that I dont remember being on the aircraft at all, just the 4 mains, 2 external, and 2 aux tanks.
Not much help I know but it may be helpful.
If you have fuel in all tanks, then the sequence for fuel feeding, assuming the pumps are on and all Xfeeds open is External, Aux, then Main.  This is achieved because the pumps are set for different pressures in the tanks, The external fuel pump pressures being the highest, and the mains the lowest. :)