Yes, there can be a problem with the 577/450 chamber. But I have never heard of the problem with the 10-X ammo.

There has been a problem with Jamison 577/450 brass. Look at this post and read the links leading over to British Militaria Forum for more details:

http://forums.nitroexpress.com/showflat.php?Cat=0&Number=105286&an=0&page=0#Post105286

If you see any of the problems reported in that post, contact Ten X and let them know. They are good people and should make it right.

What I think your real problem is more to the standard of modern brass having a flat shoulder and the original chamber having a ogee shoulder. The arc of the lower radius of the shoulder binds against the flat shoulder and will not allow the case to fully seat. If you were using foil cases as was intended for this chamber, you wouldn't have a problem. I have copies several different original chamber drawings for this cartridge (Birmingham Proof house holds 37 different drawings)and is a wide variation from drawing to drawing in 2 shoulder radius found in that shoulder.

Oh there is a fix. Get a set of Lee Dies. These were designed for the very tightest Martini chamber and elimiates this problem 99% of the time.