I have 500 RP nickeled 45 Colt cases that I have been using for a couple of years or so. No issues like this. However, I learned early on that to load 45 Colt cases without sizing difficulties; I needed to lube the cases. I learned this after getting some cases stuck and having issues with plating being scraped off. After I started case lubing, no further issues and the cases go right through the sizing die. Even non-plated cases are difficult to get through the sizing operation without lube.
Try case lubing and see if it resolves your problems.