I've loaded hundreds of the 2 1/2 16 g RST Hulls. I use a Mec 600 jr setup for 10 g to de-prime and prime and load shot and powder using a Lee Load All for the shot and powder and finish by roll crimping use a drill press.
I think part of the problem may be your use of popped popcorn to get the right volume and when you go to roll crimp the popcorn deforms/compresses in an unpredictable manner.
I'd suggest a different wad and/or adding fiber wads and cards to get the right height. Otherwise try a different filler that is more predictable, such as grits, cream of wheat, etc
