I ran into the same problem. Happens with expanders that are extra large and deep. You just get a lot of stiction and galling, even with dry tumbled cases and lube.
I added a relief groove down the middle of the plug just to break up the area and to hold a bit of lube. It's much improved. I just chucked the plug in a lathe and touched it somewhere in the middle with a cutoff disc. If you have a steady hand, you might be able to make 2 or 3 grooves. Be sure to at least leave full OD at the tip and base, and you should be ok.
This reminds me of why I think grooveless bullet molds for PC might be a bad idea. The grooves might help reduce friction and give a place for displaced lead and debris to collect. Copper jacketed bullets are designed to reach the land but not completely fill the grooves. There's some extra room in there.