I never figured out what causes it, but the old-fashioned cure for
the condition you describe is good ole Coca Cola.Get a can or two
(depending on the size of your tumbler). Open it up and let it stand
until there is no more fizz. Clean out the tumbler, run clear water
through the shot, return the shot to the tumbler and add the
fizzless Coke. Run the tumbler for a full cycle or longer. When you
empty it out and drain it, your shot should be bright and shiny
again. If it isn’t, but shows improvement, repeat the process.
It’s worked for me on the same rotary tumbler for 20+ years. If it
doesn’t work, try changing your detergent or using bottled water.
I’ve had to change detergents a couple of times when they failed to
do the job. Don’t give up. It’s curable. It may never be as
brilliant as when you bought it, but it’ll be clean and shiny and
gray and not slimy and perfectly functional.