Seems to me the simple ones are totally mechanical, like a pair of binoculars. Maybe something got knocked loose. But my most likely suspect is that your solution was not totally homogenous. Big volume with hydrometer in it, measuring gravity of the whole volume... Then tiny variable sample on the refractometer.
Hence why I'll be doing mine on a stir plate. 10 min stir, measure, add , 10 min stir etc
Did you recalibrate before each sample?