Rich,
Because the number of possible ways anything can go wrong on any system it is impossible to accurate explain, for any given system, why Windows Update has not chosen to do a Feature Update. Sometimes, this is the result of a bug.
Microsoft always "declares" when a given Feature Update is considered to be available for ALL systems and can be safely installed on any that may have not updated, for whatever reason, after that. See the
Windows Update Release Dashboard. The last Feature update declared to be "ready for broad deployment," the code phrase for, "ready for anyone and everyone," is Version 1909. See the
Known issues and notifications section for any given version.
Any version where that is stated can be safely applied, whether or not Windows Update did it by itself, and when that status is reached if Windows Update hasn't applied it already then there is likely something wrong with Windows Update on the machine in question. If it's not noted as "ready for broad deployment" that means that the roll out is not, as yet, complete. There are plenty of machines still on 1909 that will likely never get 2004, as it's not "ready for broad deployment," and its far more likely to be skipped in favor of 20H2 when the time comes for that 1909 machine to update, as 20H2 is a trivial update on top of 2004.
I don't quite understand why Windows Update answers that cannot be precisely answered strike you as "frightfully unprofessional" when scads of answers in this business are equally or even more vague. "Did you power cycle it?," along with the attendant, "That often fixes it," is a truism of our business.
After being in this business for decades now I care a lot less about having any precise answer about the root cause of any issue, because very often it's impossible to know, period. It's frightfully unprofessional if I cannot come up with a fix, and while I won't say that never happens (for any of us), it seldom happens for me as I try to assess "fixability" before even agreeing to take on a given job.
