Z3 Compact from China - will not upgrade from its loaded 4.4.4

[AC Question]

Just got a Z3 compact from China - all seems very well. It is, however, on KitKat 4.4.4 - which i can live with, but... although i know that the Z3 can have Marshmallow the phone is not upgrading (i'm told, by the phone that it's up to date).
So what could the reason that it will not upgrade it's self?
I have seen that its possible to do any upgrade myself?
Thank you
David