Greetings,
Are you wet tumbling using the "LemiShine" product?
You can get the brass too clean such that it will stick to the expander plug and Taper Crimp die.
Have a look at the expander to see if there are any streaks of brass. If so, remove the plug, chuck it in a power drill / lathe and polish the brass away using a piece of fine ScotchBrite or fine Steel wool and oil.
Back to the brass. After washing, fill the tub with water again and add a capful of liquid car wash (TurtleWax) and tumble a few minutes. Then drain and dry. Do not wash off the TurtleWax. This will act as a lubricant for the expander.
Cheers,
Dave