Why does my scale not read reliably or return to zero reliably?

I found that the load plate (top part) of the scales had become slightly twisted in its mounting (through the strain gauge) to the base of the scales. This was causing the load plate to rub slightly on the base and make the readings unpredictable. By slackening the two screws that mount the load plate to the strain gauge, and realigning, I was able to get the scales working perfectly again.