Sometimes the castled nut sp? is a different height and the slots in it are either too high, or as in your case, too low.
Check if there is one more suited to your application.
Also, with the nut off, see if any of the threads on the stud have been deformed, i.e. stretched, it will usually show up where the threads end at the larger shank portion. If not, you should be OK to go.
