8 ply is hard enough, but if it's 8 ply rag, it's even harder. I've taken my 8 ply rag board to proshops with very expensive automated cutter that cut in multiple passes and they still can't cut the 8 ply.
I have a logan 650 ( I think ) and with most careful cutting, I can do 8 ply, but I still have issues from time to time.
8 ply, is just difficult.
Best of luck,