Maybe your shell threading is gunked up with corrosion or solidified grease. I would start by going in there with some degreaser or kerosene and a toothbrush or (better yet) a small brass-bristled brush (look at harbor freight to find these for cheeeeep). Once you scrup some gunk out of the threads and get them nice and shiny again, do the same to the threads on the cup and see if it goes together any easier. It would be wise to put some fresh grease on the threads before reassembling the final time so you don't have corrosion or seizing problems the next time you need to take it apart.

If that doesn't work, you probably need to get the threads chased. Maybe your shell is slightly ovalized or has a slight dent or something that would cause the cup to not want to thread in. Retapping the threads with the proper tool should at least partially correct this problem to the point that you can adjust the cup without a cheater bar.