I don't own those particular scales, but you must realize that there's a LOT more going on inside a digital scale than there is a balance beam scale. There are electrical components that need to have a constant voltage applied. There is a membrane that must be held at constant temperature. There are also mechanical pivots which must over come certain amounts of friction. Then on better digital scales, there is software to make sense of all these variations.
You didn't tell us much about the load range you're trying to measure, but if the issue is low end friction affecting light pistol loads (3-12 grains), it can be a huge help to "zero" the scale with a dime under the pan. The added weight helps to get the scale out of the "sticky" zone.
If your loads are 20 grains and higher, then check the place where the scale sits to make sure it's not rocking or teeter-tottering even a tiny bit. A shelf attached to the wall behind the bench, rather than on the bench top, can also help.
You might also place some temporary cardboard barriers around the sides and back to make sure no breezes are blowing on the unit, causing temperature drift. One overhead A/C vent is all it takes.
Lastly, put the scale in a place far away from florescent lamps and computer monitors due to the magnetic fields involved.
Hope this helps.