Air currents, external vibrations, static electricity, electric frequencies from florescent lights and other things can all effect a digital scales. Air flow and vibrations will also affect a beam scales. Another that can and does affect both are magnetic currents or fields. My scales went ( both styles) went weird occasionally until I realized they were setting in the draft from a register for the heat and air conditioning.
A good stable solid surface, on the digitals a anti vibration pad, I use a leveling pad with 3 feet under the scales for a no rock level surface. Under my gem pro is the anti vibration pad that came with it. deflector on registers in the room, to reduce redirect air currents.
Last as with any precision insterment is to check zero and calibration before use. A set of check weights goes a long ways to knowing what you have and what its doing. I also check at the weight I'm going to be weighing or as close to it as possible. A 50 grn, 10 grn and 5 grn make a 65 grn weight. I can normally get with in .5 grns of the desired weight. ( ive been going to make a set of 1.1-1.5 grn weights to make exact easier.
Keep the scales clean and dust free and clean. Ive corrected several scales that were giving issues simply with canned air and a good cleaning.