I think what you mentioned here is the whitish precipitate, which I take as
undissolved silver, and which does dissolve with the addition of tartaric
acid. However, the silvery colored plated silver is a different matter; no
matter how much tartaric acid is added (I even added some ammonia), it
would not disappear. This loss of silver from the solution definitely would
not help but reduce print density, and that would explain why the freshly
mixed solution would give greater density. By the way, this happens with
both the solution from we bought from you and the ones I mixed from scratch.