What’s interesting is that because the new Burndy connectors fit as they should, the others must be slightly different, perhaps slightly incorrectly made. It seems there may be some QC learning for Naim here.
You’d think it would be really easy to test the Burndies for fit before putting them in the box, but who are we to say? As the saying goes, they should just work. Anyway, your problem is resolved. Hurrah.
I have found that my S-XPS Burndy can be quite difficult to lock the rings if there’s some twisting or torsion in the cable itself. Other than that it’s usually just a firm grip and twist on the connectors with the cable in a fairly relaxed state.