The 9mm case is one of the harder cases to resize. It is a tapered case and as a result more of the case is in contact with the tapered carbide sizing ring creating more friction.
If your cases are new brass or clean shiny brass then they are even harder to size. If you size the cases before you clean them the job will go a lot easier. To make it easier still, put the cases in an oily rag and massage them between your hands like a bag of marbles. The light coating of oil will make the resizing almost effortless and you won't get any stuck cases. This works for clean or dirty brass.
