The way these work is that there are two sensors. The average of the two is the fill level. So for instance if one is all the way full and the other is empty then the gauge will read half tank. There is a siphon tube that sucks gas out of one side into the other as fuel is pumped out and returned. It is most likely that the siphon tubes are damaged or kinked. I have also seen these gas tanks need to be replaced. A good way to diagnose is that the one side of the tank will always be full. When the technician drains it they should be very close to even fill level.

Good luck