[MENTION=131875]andy k[/MENTION] - OK, good. Now I feel I understand. Hey, often problems are difficult to articulate, don't feel bad about your description!

This only leads to more questions!

How difficult is it to insert the stem into the steerer tube? Resistance/no resistance
Is there a split in the threaded part of the steerer tube? Unlikely but if there is a groove for a tab on the washer, could be a crack in the steerer that expands when the stem is inserted.
Does the locknut even start thread engagement? If so, is there as possibility that the locknut is being forced to cross thread due to forces from the stem when it only goes on so far? - stem axis not in line with steerer axis.

What is the stem retention configuration? Tapered or beveled expander? Is it possible the stem is flared at the end?

Am I coming up with 1000 words to replace a picture?