The neglected genius, unrecognized during his lifetime is a standard concept in modern discourse. Yet good examples are exceedingly rare.
(Or unrecognized during her lifetime, since it has been a tenet of feminist theory that there were/ are numerous unrecognized female geniuses. However, I don't think that forty years of feminist - ahem - scholarship has come up with a single new example.)
Of course it takes a while, usually a matter of decades, for somebody to become really famous - but the genius who was essentially unknown and disregarded during their lifetime and only emerged after death is pretty rare considering the cultural currency.
In classical music, among the certainly first rate, there is probably only Schubert (the example comes from Karl Popper's autobiography).
In poetry, perhaps Emily Dickinson would count; probably William Blake (although well known for his art work).
But some of the supposed examples, such as Van Gogh or Mozart are simply untrue - Van Gogh was well known (and sold his work) and Mozart extremely famous during their lives - dying in insanity or poverty is not the same as being unknown; and of course when somebody dies young there has not been enough time for their reputation to be consolidated.
Reputations rise and fall, of course - and there is dishonest boosting and denigration - but in general geniuses are (or have been) known and recognized by their contemporaries - although not necessarily given pride of place.
Fashionable and powerful figures are always in evidence - e.g. Spohr in classical music seems to have been regarded as first rate in Victorian times while Mozart was neglected for a while (seen as a composer of pleasant trifles - rather as we might regard J.C. or C.P.E Bach).
But while trajectories are various, the specific notion of an obscure and neglected genius who lived a full lifespan in the wilderness and was only recognized by posterity is, in fact, a very rare bird.