Originally Posted by fixedpip
Other options:
You could always get the threads chased to clean 'em up.
Get busy with a toothbrush and some cleaner on the threads to flush out all the dirt.
The first things that jumped in my my mind are: The theads are stopping it or the bearings/races are stopping it. These are both fairly easy to check for though.
Try screwing in the cup without the spindle or bearings. Does it go in any further than when you try to install everything? If so, the spindle/bearings are your problem. If not, it's the threads. Maybe this will help with the diagnosis?