This might be caused by the default aircraft you use in FSX/P3D. In FSX/SE, I use the default C172, in a cold and dark setting, but with the Avionics On. Also make sure that the Fuel Selector in the C172 is set to ALL. It is set to ALL by default, but it might not ber set to all if a different aircraft was used prior to loading the default C172 saving the default flight.

In P3Dv3, I use the default Beech Baron 58, with the same settings I use in FSX/SE, and in P3Dv4.4 I use the J3 Cub, and again use the same settings I use for FSX/SE/P3Dv3.

While I believe the 737 will still receive fuel if the Left and Right (FWD & AFT) fuel pumps are OFF, it's worth making sure that they are ON just in case they don't.

Note. Seeing as you didn't say which flight sim you were flying the 737 in, I covered the settings I use for FSX/SE, P3Dv3, and the latest version of P3Dv4.4, so that it might help you solve your problem. ;)