Tumbling sterling flat pieces leaving little pits?

I’m trying to find the best method for tumbling flat sterling pieces
that have been hand stamped. I use the Lortone and mixed stainless
steel shot. It leaves tiny little dots/pings all over them. Any
suggestions? I’ve tried filling with water, less water, without the
pins, etc. I spoke with Lortone and Rio and they have no idea either.
A different media? Maybe this is normal?

Thank you!

what is the gauge of your sheet? i use the same system and media and
haven’t noticed anything like that. maybe your silver is quite thin?