When I first got mine, the reading were erratic. I could test the same sample three times and get three readings.
Two things I learned, one, when the instruction say fill the reservoir full, it means completely full, not just covering the little glass window and two, after filling the reservoir wait 30 secs and let the temp stabilize before hitting the READ button.
Using the default correction factor built into ProMash, I am always within 1 gravity point of my hydrometer.