I’ve run into a few similar challenges, although in almost all cases I’ve found that searching on Tropicos, BHL and Google Scholar will turn up some reference to a plant species if it was validly published.
But there are a few authors I come across who published copious novelties in journals with a tiny circulation that have never been digitized. If these weren’t reviewed, indexed or addressed by later researchers, a species may seem to be unpublished when it’s merely unappreciated.
Sometimes, working on these conundrums, it’s a relief to come across the abbreviation “sp. nov. ined.” and know that the taxon can be formally debunked (unless it was published for real at a later date, of course).