Pixhawk Orange Cube Drone experiences a drop in altitude during flight

During the flight for about three hours, the drone’s altitude did not rise even when the pilot’s throttle was raised to the maximum, but gradually decreased to the point where it reached the ground. Why is the altitude going down so automatically all of a sudden?

Could be many reasons. Battery is the first one that comes to mind, but barometer drift over a long flight is also possible. Just guesses. Post the .bin log file from the flight and we can help more.