I had the same problem on several occasions and found out my local
water supply was the culprit.
I started using bottled water and haven't had a problem since. The
solution I found was to disconnect and thoroughly clean every part
of the flow-thru system (bucket, pump, hoses), scrub the tumbler
bowl, and clean and rinse the shot well. Without reconnecting the
flow-thru, run the Super Sunsheen liquid (and bottled water) with
the shot for a few hrs. Then, drain the tumbler and run bottled
water through the shot until it drains clean. Reconnect your
flow-thru with a fresh batch of solution, and your silver should
come out shiny again.
Hope this helps.