Are you correcting for temperature? Solutions get less dense as they get warmer, so as your sample cools it gets denser (higher gravity.) You have to take the temperature of your sample and correct the reading to the temperature your hydrometer is calibrated at (either 59°F or 68°F usually.) Your hydrometer should have come with a chart showing how much to add or subtract at any other temperature to get the actual gravity, and should be marked showing the calibration temperature. If you don't have a chart, search for hydrometer correction, there are calculators out there. The actual gravity will not have changed, just the apparent reading at different temperatures.