I’m no expert, but I’ve tried this method for stripping silver off of plated silverware.
For some reason I found that it starts well, but as more and more material goes through the solution, the stripping doesn’t actually happen at all. For some reason I’ve been experiencing copper plating onto the silver plating!
So it gives a false impression that the silver was removed. Because there is a thin layer copper all over the item, but when I look more carefully and sand down a bit of the surface, the silver is still very much in tact…underneath the copper! I’m not sure why this is happening, because I’ve seen many videos of guys using the 3:1 conc. sulfuric and nitric acid mix, with good success.
But this may be why you’re not getting precipitation of Silver…because most of it is still on the items you processed, underneath the deceptive layer of copper?
Just a thought for you though I might be wrong, but try sanding some of the processed items with fine sand paper, and see if it reveals silver underneath the copper?
You can strip silver with plain hot sulfuric acid too. It is slower than with nitric/sulfuric, and also probably more dangerous, but it also works OK. Not that much copper is actually dissolved as with combination of both acids.
As 4metals pointed out, cyanide is far the cheapest way of stripping silver, if you have license to purchase and work with cyanide on larger scale. Anyway, all risks accompanied with cyanide are there, and it is very very dangerous to do (for you and anyone in proximity), if you do not have special training and diligence regarding work with cyanide on semi-industrial scale.
There are other methods. Water cell was described here on the forum pretty well. There is also possibility of electro-stripping using ammonia as electrolyte, but this is rather non-selective and potentially dangerous. But it works fast. Silver is then reduced using common methods which work in alkaline solutions (like sodium borohydride) + harvested from cathodes, depending on how it actually been performed.
Then, if you have pure copper as base metal, you can melt it into anodes and electrorefine the copper out, leaving silver in the slimes (this is closest to what actual smelters/refineries do with raw copper). If material is brass/bronze, you can pyrometallurgically remove zinc/tin, and then electrorefine.
---------------
I also seen this rather weird behaviour, not only with copper coated silver, but also with copper coated gold. It can be result in difference of potential of the solution with residual metal on the material, I am not sure, this is just a guess. But when for example all of the base metals are leached from the gold, no more coating is observed afterwards.
