It's a mechanical scale. There has to be something wrong which can be seen.
These things don't have strain gauges, batteries, etc. They are as reliable as gravity, if they are clean.
What about the knife edges? Nice and sharp on both sides? All it takes is a burr and it won't pivot correctly.
Agates aren't cracked? They should be "floating" in their pockets.
Copper vane isn't bent and touching? (I think you already said nothing is touching, but this kind of rides "behind" everything else.)
If all the parts are OK, it has to work. Don't give up on it...
.30-06 Springfield: 100 yrs + and still going strong