Thanks, Wards. Your reply may be most useful, and it is interesting. I'll have stuff to try this evening when I get home.
To be sure, the 2 phones that I mentioned in other recent posts are gone, taken back by the dealer. So, I am talking of 3 other similar (and new) ones, two of which replace the ones that I returned and are working, just not upgrading. I ordered these for other folks.
It is very useful that you noted that you did proceed despite no initial recognition by WDRT, which I always did, as well. In my case, however, despite the "flash lightning" mark showing, they are behaving just like the returned ones as far as not being recognized...
...YET these still start a process of updating. It just fails to update shortly after, and this is so reported on screen.
So, PERHAPS -- if I am lucky enough -- it is just the cable. It may be that I always used the same one. It is the only thing I failed to check...it being new and all.
Then, there is the other possibility; that useful bit that you mention about Windows 10 Pro. All my PC's are on Win 10 Pro of various types (levels of Win 10 upgrade) and platforms. I'll take my small, old & slow crappy Samsung laptop home. I keep it in a drawer here at the office and it has plain Win 10.
Many thanks for taking the time. I'll report later.