Film historian and documentarian Kevin Brownlow, who’s been making movies about movie history for more than 30 years now, and also has been restoring, scoring and re-releasing silent films.
Rather than Bach, check out classical composer Johann David Heinichen, whose music was nearly as good, but who has been completely forgotton except to music historians. The problem was that he did his work on commission for the Crown Prince of Dresden, who kept all his work locked away.
I’m so glad someone mentioned Skip James. 
Mr. Alpheus Green, high school mathematics teacher.
Sure, NOW! But the poor guy died at an advanced age in extreme poverty, and missed out on the huge amounts of money and fame Edison made by ripping him off–poor bugger got screwed out of just about everything during his lifetime. Cold comfort when you only get recognized decades after your death, and even then only by the cognoscenti.
I’d say the OP title “Unacknowledged Greatness Over Decades” is perfectly apropos and I stand by my nomination of Nikola as an unheralded, unrecognized genius.
Proof perhaps that it never pays to be multitalented. :dubious: