Isa,
zero the scale.
I'm sure combilift may be able to help but the mfg of the scale should have supplied a manual for that scale, if not you can contact them also.
Most scale's have a zero button or in some cases it may require a re-calibration as was suggested which would be included in the operators manual.
Do you have a brand and model number of the scale?
Maybe we can figure this out on here so others will know also :o)