These are almost certainly maintenance and adjustment issues. First rinse the cell with a warm 1% caustic such as Alconox (amazon) and let it set 3-5 min then flush thoroughly with distilled water (grocery store) ....and then flush again and again. Wait at least an hour, then perform a water check. If fails, try again. If you are certain the cell is clean, follow the prompts to perform an adjustment. Run a major brand stripped vodka and should be very close. Triple Platinum is sold everywhere, I believe, and will measure 40.00 %v/v (using Ethanol AOAC @ 60 K method).
Run water checks daily (min). You should only need to adjust the instrument (with distilled water) a few times/yr (which is why many stop checking).
My DMA has been hit by a forklift and traveled many miles in my truck bed and always measures right-on. I use many of our customer's brands basically as standards. They almost all bottle at slightly different proofs, but they are always the same, or will be on a properly maintained DMA.