Response below if you have sender and gauge like a RB/late model B where the gas pickup is part of the sender. Apologies if you have an earlier model sender/gauge setup, not sure this is applicable.
Did you change out the sender? On later RB senders, there is a tang that controls the upper sender arm movement. As stated in the library, don't bend the sender arm, but you can adjust the tang to allow more upper movement on the arm to register fuller tank - lower ohms for fuller gauge reading. If you did not change the sender, maybe you have degraded the ground connection at the sender somehow or for whatever reason you may have to adjust the gauge.
When I replaced the sender on my B, could never get the gauge to read full tank, only about 3/4. I ended up adjusting the fuel gauge. I powered the gauge off of the dash power, normal connection (from the stabilizer at 10V) with a full tank and adjusted to full. When I killed power, then adjusted for empty tank. I also disconnected power to my ignition coil while doing this since I believe the coil would be damaged otherwise. The fuel gauge setting seems to be a little delicate, so be careful and patient. Also, the full/empty range settings are tied together, so don't over adjust for full.