Why will my scale not read accurately?

I had a similar problem with a Wheatstone bridge strain gauge based set of scales, after the device had been dropped.

After taking the scales apart I determined that the strain gauge and electronics were working fine by placing the load plate (top part) with the strain gauge still attached upside down, and then placing known loads on the underside of the strain gauge. This gave correct and reliable readings.

Therefore, I deduced that there was a mechanical problem only present when the device was fully assembled. After checking that the wiring to the Wheatstone bridge was not getting trapped between the base and the load plate, I noticed that the strain gauge was actually mounted slightly twisted relative to the load plate. This was causing the load plate to rub slightly against the base when a load was applied.

Loosening, re-aligning and re-tightening of the strain gauge as mounted to the load plate has fixed the problem.