Yep I have had some cases do that in the past as well. IT's a bit time consuming, but if you want to clean them up really nice, you can do it with a drill press or even a cordless and a short piece of wooden dowel sanded to fit snug inside the cases, some Flitz and an old shop towel.
Just shove the case on the dowel, fire up the drill at a nice slow speed, and polish away. It usually has come off pretty easily for me in the past, and the cases polish right up.
The draw back I had was I would get a cramp in my thumb after about a dozen or so cases. So switch hands often while gently squeezing he towel around the spinning cases.
You might not get every last bit, but it will sure make them look a bit nicer.
Mike / TX