I have a kitchen scale that doesn't seem to be working properly. I have tested it by putting already weighed packaged goods on it and it comes up short. For example, 454 grams of butter is shown as 360 grams. A 300 g package of chocolate chips shows as 260, etc. This scale is battery operated and I have put in a new battery. I have also zeroed the scale first. Nothing seems to works. Any ideas of what I can do to get the proper weight?