Q: Fuel tank gauge registered inaccurately & repair did not work

The fuel tank gauge of our car registers inaccurately. I took the car to the dealer and he reset the computer, then told me that all was okay and I assumed that the problem had been solved. Usually, I fill up the tank when it is about ¼ or above. About 3 months later, the tank ran below ¼ and the gauge when up. Since it is obvious that the problem was not fixed, can you tell me what would solve this problem?