Harrison M300 lathe - Single point threading part 2: pitch mismatch mystery

Follow up video to Harrison M300 lathe – Experimenting w... where I seemed to be able to cut an M10 1.25mm pitch external thread ok, but I found I could not wind one a 28 mm long nut all the way onto it. I continue the investigation as to why, but without a conclusive resolution.

* The nut is ok: it's cut with a 1.25 mm tap which runs through it ok from both ends.
* It's not a problem of taper on the O/D of the external thread: it was actually slightly thinner where it jammed up.